Bottoms up (INFORMAL) Used to express friendly feelings towards one's companions before drinking. "Bottoms up! Luke enjoyed a glass of white wine"

Honestly, without further contextual information, I can't be sure what exactly you mean. If I had to guess, you could mean drink the contents of a glass, aka "drink a glass" aka "down a glass" - this is what the two replies before mine were talking about, and this appears to be the main explanation in Google. But there is a chance you could also mean "discard the contents of a glass" (eg, tip it down the drain) aka "empty a glass". There's also a chance you could mean, to siphon the contents of a glass, ie, to move the liquid somewhere, eg, via a tube into another glass. It really depends on the context, which isn't provided here yet. So to be absolutely sure what you mean, I would want to know where you heard this phrase. I'd need a bit more context.

To drink everything (in the glass)