when you unscrew the cap for your bike tire, you'll see something that looks like a metal stem in the middle--that's part of the valve core. you have to remove the valve core if you want to add bike slime. bike slime is this green liquid that seals small punctures so your tires don't deflate when you accidentally run over thorns while riding. there's probably a special bike tool you can use, but tweezers work pretty well too. you have to get the tweezers pretty far in, then unscrew it as the valve core is threaded.
oh and whatever you do, don't use needle nose pliers to just yank out the "stem."