what is the meaning of these sentences? "You know where the drop is going down."

Solution 1:

By You know where the drop is going down. the speaker is referring to the location of where the exchange of an item / package is going to happen. Either that or he/she is referring to the location where some person / item / package is actually going to be dropped from the air via helicopter / airplane of sorts. Although I suspect it to be the former.

This language is commonly found in the dialect of thiefs, drug dealers and such people or even sometimes police.