The core of Parkour is the idea of using movement as a tool to help you get past all the obstacles you face.
Explaining Parkour is all about finding out which parts (of the definition) they don't understand and helping them to understand.
There are lots of things that they may need help understanding. The common ones are movement itself, the concept of training/improving, what we physically do, and how we approach it mentally.
If they don't understand what movement is, then the best way to explain it is usually something like "Moving from point A to point B as efficiently and effectively as possible".
If they don't understand what you mean by helping you get past obstacles then you can explain it well using such ideas as "Helping you get past the obstacles you face in life" and "Helping you get stronger in many different ways".
If they don't understand what this involves physically then the simplest way to begin to explain it is to say "We try and move past the obstacles in our environment".
If they don't understand the mental approach, the best way to explain it to most people is by using ideas like "We have to try and think positively at all times" and "There is always a way forwards" and "We keep believing that we can continue making progress".
As a general rule I find it best not to try and refer to anything the person is already familiar with. At the moment, if something is famous enough for someone who isn't a practitioner to know about then it's almost certainly a bad example. People find it easier to understand something new than to change their existing understanding of something they think they know about already.
The only way you're going to be able to explain Parkour well is if you have a comprehensive and detailed understanding of it yourself. That way you can change how you explain it to suit the person you're explaining it to. If you simply repeat the same phrases all the time then every person you explain it to is going to understand it how you understand it, instead of how they need to understand it.