At 8 PM Eastern tomorrow night- or, more to the point, 7:59:59 PM Eastern, Earth is going to experience a leap second. One Earth day is thought of as 24 hours in a day, 60 minutes in an hour and 60 seconds in a minute- or, 86,400 seconds in a day to turn it all around or throw it all away. However, it's not entirely that simple. Nature has a way of not always adhering to nice, clean, convenient-to-simple-human-math numbers all the time.
Way back when, the time was determined simply by Earth's rotation around the sun, with the definition of hours and minutes and seconds and days and years being 'however long it took to do this much of a rotation'. Back in 2010, it was mentioned here that the modern definition of a second is "the
duration of 9,192,631,770 periods of the radiation corresponding to the
transition between the two hyperfine levels of the ground state of the
caesium 133 atom." That's been the official definition since 1967, and has been used to measure time with atomic clocks that use caesium atoms for this purpose. Caesium atoms, of course, have nothing to do with the Earth's rotation.
Since then, scientists noticed the Earth's rotation slowing a tad. 2 milliseconds per day, to be exact, mainly due to the tides sloshing around. When Earth's pace gets too far away from the time on the atomic clocks, the solution is to add a leap second; they don't want the difference getting any more stark than 0.9 seconds. That decision's made about six months in advance, usually when the difference is at about 0.6 seconds, and the leap second is always scheduled for June 30 or December 31. This is going to be the 25th leap second overall; the first came in 1972. (There are provisions to take a second back if that should have to happen, but so far it hasn't.)
They're considered somewhat annoying in the scientific community, so much so that some would prefer they be abolished in favor of leap hours: wait several hundred years until the clocks and Earth are disagreeing by the better part of an hour and leave our great-great-great-great-etc. grandchildren to do it all in one go. That debate is still hammering itself out.
For now, though, tomorrow you have one extra second to do whatever you want with. What will you do with it after you make your alarm clock tick all the way around and make it read one second earlier? Which should only take you like a minute or so.