Anything rocky is covered with rocks, like a rocky beach or hillside. You can also use rocky for something that moves in a bumpy or uneven manner, like a rocky ride in a Jeep.
There's a figurative sense of rocky that means "difficult, rough, or up-and-down" like your rocky relationship with your sister or a struggling business's rocky future. This meaning comes from a combination of the "uneven and covered with rocks" definition and the verb rock, or "sway back and forth." The Rocky Mountains, which aren't actually all that rocky, were named for the local Native American people, called "the stone Sioux" by their Cree neighbors.