A leap second is a positive or negative 1-second adjustment to the Coordinated Universal Time (UTC) time scale that keeps it close to mean solar time.
Leap seconds are necessary partly because the length of the mean solar day is very slowly increasing, and partly because the atomic, SI second, when adopted, was already a little shorter than the current value of the second of mean solar time.
After 23:59:59 UTC, a positive leap second at 23:59:60 would be counted, before the clock indicates 00:00:00 of the next day. Negative leap seconds are also possible, should the Earth’s rotation become slightly faster — in which case, 23:59:58 would be followed directly by 00:00:00 — but they have not yet been used.
Over long time periods, leap seconds must be added at an ever increasing rate.
The red line is the deviation of the day length since the introduction of leap seconds.