A propensity is a natural tendency to behave in a certain way. We all have propensities — things we tend to do. Dogs have a propensity to bark, and many people have a propensity for getting annoyed by it.
If you have a propensity for something, then it's something that comes naturally to you or something you just do a lot. Some people have a propensity to laugh. Other people have a propensity for making others laugh, or for being generous, or for getting angry. It's hard to change your propensities. Sometimes a propensity is a bad thing, as in a criminal with a propensity for theft or murder.