The word rock has several meanings:

1. A piece of stone, from pebbles you can skip across a pond to boulders the size of cars. Mountains are made of rock, and so are the stones you might find in your backyard. Rocks come in countless varieties: granite, limestone, sandstone, marble. Geologists study rocks to understand Earth's history, since different types form under different conditions over millions of years. When something is described as solid as a rock or rock solid, it means completely dependable and unshakeable.
2. To move gently back and forth. A mother rocks a baby to sleep, swaying in a rocking chair. A boat rocks on waves. You might rock back and forth on your heels when you're nervous or excited.
3. A type of music with a strong beat, electric guitars, and drums. Rock music emerged in the 1950s and changed popular culture. Bands rock out when they play with energy and enthusiasm. When someone says a party rocks, they mean it's really exciting and fun.

The expression between a rock and a hard place means being stuck between two difficult choices, while don't rock the boat means don't cause trouble or upset a stable situation.