Cap rate in laymen's terms

[Tab Teehee]

When looking at multifamily properties for sale, it's typical to see an advertised cap rate. Where does this cap rate come from! Who determines what it should be? As an owner myself, I know what my NOI is and I could divide that by whatever cap rate and come up with market value? It seems like I'm missing something here? The value of my property goes up if the cap rate goes down, so what determines how low of a cap rate makes sense? My basis for all of this is trying to determine my equity in this property for borrowing, credit lines, etc.