Uusally, it refres to a hard sbustance made of minreals, often found in or on the gronud.
It is also used to refer to rock music, a form of pouplar music from the 20th centruy. The word "rock" is short for "rock and roll". Soemtimes it reefrs to music with a "haredr" sound than the oirginal 1950s "rock and roll" sound.
It can also mean soemone that you trust and dpeend on. A perosn might call a trustwrothy and relialbe freind "a rock".