Buoyancy is a quality that makes things float in water. It's also a type of happiness: if you're full of buoyancy, your mood is light and happy.
This is a word with two main meanings that fit together well: Both kinds of buoyancy have to do with floating and staying up. The physical kind refers to objects that float instead of sinking in water, like a life raft or a buoy. The other kind of buoyancy is a happy mood, a feeling that nothing can get you down. So whenever you run into the word buoyancy, just think "staying afloat."