Don't is a contraction of “do not,” used to express something that should not happen or something that isn't happening. When your teacher says don't run in the hallways, she means you should not run. When you tell a friend I don't like broccoli, you're saying you do not like it.

Contractions like don't make speech and writing flow more naturally. Compare “do not touch that hot stove” with “don't touch that hot stove.” Both mean the same thing, but the second sounds more like how people actually talk.

Don't appears in countless everyday phrases. Don't worry means stop feeling anxious. Don't mention it is a polite way to say “you're welcome.” Don't count your chickens before they hatch warns against assuming success too early.

In writing, don't fits casual contexts like emails to friends or dialogue in stories. Formal writing sometimes prefers “do not” instead. When emphasizing something strongly, people often write “DO NOT” rather than “don't” because separating the words adds force: “DO NOT press that button!”

The related contraction doesn't means “does not” and works the same way with different subjects. You say I don't know but he doesn't know.