Did you write one? That is, did you define that function somewhere else?

It's a user-defined method, not one that is part of Processing.

You can find the classes & methods that are part of Processing here: http://processing.org/reference/

Check the sketch where you got this code from for the custom method isOver().

If you want to check if the mouse is over a rect (or a circle) see #17 of my list of life-saving tips for Processing.

Thank you for both of you. The reason why It doesn't exist because I didn't define it. The problem is sorted, thanks again. 

" is there any way to make the sketch runs faster?"
Probably. You have to optimize it and to avoid some common pitfalls.