Come June 30, there will be an extra second in the day, making the hour before midnight 3,601 seconds long.
Like leap years, leap seconds occur to bring our timing devices in line with nature. We have leap years because it doesn't take the Earth exactly 365 days to orbit the sun. It would be handy if our spin and orbit were nicely aligned, but the world is not built around us. Ancient instruments provided an estimate of 365.25 days to the year, which made for a leap year every four years. The realization that the year was nearly 11 minutes shorter, depending on the measure used, meant a round quarter was too easy, leading the Gregorian calender to leave out three leap years every four centuries.
The need for a leap second is a little different. The Earth's rate of spin changes ever so slightly in response to the gravitational effect of other bodies, particularly the moon.
Most days we spin a little bit slower than the one before, an average loss of 0.002 seconds/day. If these changes were allowed to add up, we would eventually find that the whole planet would experience the midnight sun, balanced, alas, by darkness at noon.
The slowing isn't uniform, so the International Earth Rotation and Reference Systems Service exists to work out just when an extra second is required and make sure clocks around the world keep together. Leap seconds are always added just before midnight Greenwich Mean Time on June 30 and December 31.
We've already had 25 of them since 1972, making the three year gap since the last longer than average. But modern technology has made these adjustments harder, not easier. Numerous leading websites were derailed by the last leap second.
These sorts of problems have led some to propose that we abolish leap seconds. After all, it will take centuries for our timing devices to get sufficiently out of sync with the planet for most of us to notice. On the other hand, do we really want to encourage our culture to do more of putting off problems to future generations?