I'll try to keep it short because it's an involved subject. Basically "stall" means when the torque converter "locks-up" under a load. With that, if you take a car with a 2,500RPM stall converter and you hold your foot on the brake hard, and hit the gas at the same time, (as if you were going to power brake), the "general" RPM in which the tires will break loose is at "about" 2,500RPM. If the converter was a 3,500RPM stall, then it would be at approximately 3,500 RPM in which it forces the tires to break loose. Keep in mind, if you put that same converter behind a nasty big block, in a car that has serious traction or is quite heavy, the torque of the engine will drive the stall speed up to a higher RPM, and an engine with less power will have the opposite effect and won't be able to stall the converter out as high. It's all relative to torque, vehicle weight, traction and rear gearing. It isn't an exact science as far as an exact stall RPM goes.
Stock cars have anywhere from 1,000 to 1,400 RPM stalls from the factory. The reason a stall converter works is simple; an engine at 1,200 RPM is only making about 80 or so horsepower (on average), but at 2,500 RPM it could be making 150 to 200HP, and obviously trying to get a car moving using 150-200HP is going to be much easier than one trying to launch at 80HP or so. Most performance engines don't make power until 3,000 or so RPM, hence why when you have a higher horsepower engine with a big cam, you need a higher stall speed so the engine is closer to it's "power band" when taking-off from the line, otherwise it will fall on its face and be a turd off the line.
Some people believe that "stall" means the car won't start moving until the engine reaches that particular RPM, and that isn't even close to being true. We build race cars with 6,000 RPM stall converters and when you put the car in gear and let it idle, it WILL roll along at 5 - 10 MPH, just like any other car will when put in gear and with the brake off. In fact, we usually cruise through the pits with the car simply in gear and idling, so if a 6,000RPM converter means that the car won't start moving UNTIL that RPM, then we'd have to have the engine wound-out to 6,000 RPM to putt along through the pits at 10-15 MPH, and that simply isn't the case. Stall means when the car is on the starting line, (with say a 4,500 RPM stall converter), and the trans brake is on, (which locks the transmission in first and reverse at the same time), and you hit full throttle, the engine will wind-up to 4,500RPM with the car just sitting there, so when you slip your finger off the trans brake button and the tranny engages out of reverse, the car instantly launches at 4,500 RPM, and a race motor at 4,500RPM is pretty close to its peak torque curve when leaving the line, hence the big wheelies you see on some drag cars or the incredibly low 60 Ft. times when traction is good. Race engines make no power at low RPM's, and are usually shy on torque, so the nastier the engine, the higher the stall speed needs to be (in general anyway). Again, cubic inches, torque, vehicle weight and gear ratio have a big factor in this. Now, this doesn't get into the "flash" effect which is where a stall converter will "flash to" when your finger comes off the trans brake button. Typical 3,000 RPM converters can flash up to 5,000 RPM or so once you've launched the car. It's a bit more complicated and I am yrying to keep this simple for you, so we'll skip that area right now.
A 6,000RPM converter behind a stock engine might not even get to 6,000RPM because the engine can't make enough torque (power) to spool-up to that RPM because of the load of the converter. The car will start rolling at a much lower RPM and may never be able to reach 6,000 RPM. This is also true for any converter from about 3,000 RPM on up. Most mild race cars (9-10 second quarter mile times) use anywhere from 3,500 stalls to 4,500 stall speeds on average. Faster cars usually use even higher stall speeds, especially in smaller cubic inch engines that don't make the torque of big displacement engines. Also, the higher the stall speed, the more heat the converter will make, from hydraulic (fluid) friction inside the converter. A high stall converter can easily make enough heat to fry a transmission if you hold it on the line long enough at full RPM if the stall speed is above 3,500RPM or so, so a good tranny cooler is essential when running ANY type of high stall converter.
Most street performance cars running your typical "performance" cam should be running a stall converter in the 2,800 - 3,000 RPM area. For hotter cars (street / strip) with bigger cams, etc. then a converter in the 3,000 - 3,500 RPM is more suited. For mild performance cars with something like a 350 HP engine, a 2,200 - 2,400 stall is about right. The basic rule of thumb is; if your engine "comes alive" at say, 3,500 RPM, then you want a stall converter with about 3,500 of stall to it. Most people tend to over cam and over carburete their cars... and those same people also tend to under stall converter their cars too, which REALLY makes it a complete turd. if they need a 3,000 RPM stall, they'll usually get something like a 2,000 RPM, which just doesn't get it off the line. You have to remember... 80% of the race is the first 100 feet. If you can't get off the line, you are going to lose the race.