I highly recommend you read the Introduction to Programming with Xojo that is linked on that same page. It answers the questions you asked and much more.
I realize the convenance of emotion in a typed response is in the imagination of the reader, but me thinks Jay believes he was dissed.
But Jay, let me tell you that is not the case, this is the most helpful forum that you will find anywhere on the internet, regardless of subject matter. HOWEVER, you need to put you questions into context, you need to show that you have made SOME effort at attempting to resolve things yourself (after all that is the basis for learning), and when all else fails you need to show us what you attempted, and why you are unsure why it didn’t work.
Refer to the Language Reference manual (commonly referred to here a “LR”), especially those sections concerning the MOUSE and its events, MOUSEDOWN, MOUSEUP … why I infer from the way you asked this question, was something you did not attempt to do.
This question has no context. “a numeric value”, as an Index? as Data Content? as a pointer to another object?
So for now I (and hopefully others), will overlook the implied attitude in the response, and hope we can be of assistance in the future.
Last thing ANY of us want is to be percieved as STACKOVERFLOW