"Droppin salt" is when someone (usually a friend) says something negative about you in front of a girl that your trying to bang. They occasionally do this unintentionally being drunk, but at times it is intentional. Your friends should be informed on what "droppin salt" is, so they know not to do it.
Conversation between Pete and Eric after "salt was dropped"
Pete: Hey Eric, why you droppin salt in my game?
E.J. : Dude, I wasn't!!!
Peter: Bro?! You were!! That girl was totally diggin me and then you walked up and said some shit like "Pete never uses condoms!!!"