There is plenty of information on the Internet about the subject, so just shortly: leap second is here to make UTC time "catch up" with atomic time (TAI). This adjustment is not extending a year for a second. It is rather to "catch up" with moving 0:00:00.
This Earth rotation delay is caused by movement of large masses in, on and about the Earth. Tectonic plates, and ocean tides caused by our Moon are messing with Earth rotation stability.
When the length of a second in IS was established in 1892 it took on average 86400 seconds per day. Since then (a century ago) day got longer. Current rate about 0.0013s. That error applied daily accumulates over a 1.5 year to a time which may justify adjustment. And that is why we need to add an extra leap second.
Example: 365*1.5*0.0013ms=0.71175s.
After we adjust for that one second the clocks will be ahead by 0.3s but Earth will catch up with that in about 0.3s/0.0013=231 days. After which we will start to fall back again. Scientists from International Earth Rotation and Reference Systems Service will monitor that till it will reach again more then 0.5 second and they will make another adjustment if necessary. Also check here: http://www.iers.org
At that rate 1.7ms per century it will take bout 5 billion years for Earth to stop spinning however it will take something like only 2 billion years and the Sun will start turning into a red giant and make the Earth inhabitable, so no worries :-)
Here is how I think the leap second is created and delivered to our systems: LINK