What does 'submarine races' mean?

Oh, dude, "submarine races" is slang for a made-up activity young couples used as an excuse to sneak off and make out in their cars back in the day. It's like saying you're going to watch the sunset when you're really just trying to get some alone time, ya know? So, next time someone invites you to the submarine races, just remember it's not about actual submarines racing underwater... unless that's your thing, I guess.