Cap Rate vs. Gross Rent Multiplier - Help please?

[Deborah Burian]

I have reviewed the forums and evidently am slow because I simply am not comprehending Cap Rate and the value of using it as a tool. For my quick look, I tend to use Gross Rent Multiplier. These seem much the same yet not identical to me. When I googled it, I got referred to Bigger Pockets :-), so I'm back to ask the community to explain it to me like I'm 3 years old. Thanks in advance.