Written by Matt Estela:

The expression node in nuke is a bit of a mystery for most people, but it's incredibly powerful. Often its used for patching data passes with simple if/else statements, but here we'll go from first principles, work up to some long forgotten high school maths to do some silly tricks, and finally put it all together to understand how a P_matte gizmo works.

If you are wanting to start learning expressions this is agood place to start.