The word buck has several meanings:

1. A male deer, antelope, or rabbit. When you see a deer with antlers in the forest, that's a buck. Female deer are called does. Bucks grow and shed their antlers every year, and during mating season they sometimes fight other bucks by locking antlers and pushing against each other.
2. A slang term for a dollar. If someone says “That costs five bucks,” they mean five dollars. Today you might hear someone say they need to earn a few extra bucks, or that they got a good deal for twenty bucks.
3. To jump and twist suddenly, like a horse trying to throw off its rider. A bucking bronco leaps and kicks, making rodeo riding so difficult. The word also describes resisting or opposing something: a student might buck the trend by doing something different from everyone else, or a senator might buck her own party by voting against her party's proposal. When someone bucks the system, they're challenging the usual way of doing things.

The phrase pass the buck means refusing to take responsibility by pushing it onto someone else, while the buck stops here means accepting final responsibility for a decision.