Hi, could someone please explain to me what 'Control Source' does?

I've read the Help on it repeatedly, and it makes my head spin... I
just can't understand what it's trying to say.

I did think I had it figured and was using it... but just now found I
can REM it out with no effect.

Here's my usage...

List1.ControlSource = "Sheet1!P3"

If the number in P3 there was 6, what should that do?

Thanks - Kirk