[expr] is one of those Max objects that can perform a multitude of more or less complex mathematical functions, depending on the arguments you give it. I am often bamboozled by it, as my math is pretty weak. I would go through the tutorial files involving any object that performs control/signal conversion, such as [mstosamps~], [gain~], [dbtoa~], [pow], and of course [expr] itself. And then you can tell me!

In the old pre-Max5 tutorials, Chapter 38 was about expr and if. You can find the old tutorials at

http://cycling74.com/downloads/older/#max462mac