Something that's early happens right at the beginning of some specific time period, or before you expect it to happen. An early party guest shows up before the party starts.
An early riser gets up with the sun every morning, and you might eat an early dinner at five in the evening. If you're in your early twenties, you're closer to 20 than 30, while the early 20th century was the time period before 1950. If a baby comes early, she's born before her expected due date. The Old English source is ærlice, "early," which comes from ær, "soon" or "ere."