O'clock is the word we add after a number when telling time by the hour. When someone says “It's three o'clock,” they mean it's exactly three hours past noon or midnight, with the minute hand pointing straight up at twelve.

The word is actually a shortened form of “of the clock,” which people said centuries ago to distinguish clock time from older ways of telling time, like sundials. Back then, clocks were new and special, so people would say “It's five of the clock” to make clear they were reading an actual clock. Over time, this got shortened to “o'clock.”

You only use o'clock for exact hours. You can say “two o'clock” or “seven o'clock,” but you wouldn't say “two thirty o'clock” or “quarter past three o'clock.” For times that aren't on the hour, you just say the numbers: “It's 2:30” or “It's quarter past three.”