means Time worked beyond your normal scheduled hours, often compensated at a higher pay rate, or the extra period played to break a tie in a game.

from A transparent compound of "over" and "time," first surfacing in the language of labor and wages in the 19th century, as the clock became the unit by which work was measured and sold. "Over" here means "beyond, in excess of," the same sense found in "overflow" and "overreach" — so "overtime" is literally the time stacked on top of the time you already owed.