In short, it means that it's random which pokemon it will evolve into. However, once you've caught a Wurmple, the one it will become is fixed in place. If it doesn't evolve into Cascoon you'll need to catch another Wurmple and try again.
"Personality Value" is a hidden number each pokemon has, which determines a whole bunch of things in the game like gender, nature, ability and whether the pokemon is shiny.
The "mod 10" bit is a mathematical term, which means that a number from 1 to 10 is derived from the personality value. The "≥ 5" bit means that if said number is greater than or equal to 5, then you get Cascoon. If it's less than 5, you get a Silcoon.