Question

What is a leap second? And how is it caused?

Answer 1

solar time is not that accurate so that it is to be measured by seconds as unit some time is time lacks behind so we have to forward our clock by some seconds to match solar time. . .

Answer 2

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time. The most recent leap second was inserted on June 30, 2012 at 23:59:60 UTC.

It caused a lot of trouble,its just basically an extra second :)