The precise reference time over the world is based on atomic clock and is maintained by all countries, this time called TAI is continous. The historical time used by people for every day life is based on observed solar time (known as UT1). The observed solar time is not stable compared to atomic clock time and varies especially because of long term slowdown in the Earth’s rotation.
In order to have both a precise time and an every day life time, Coordinated Universal Time (UTC) has been created. It is based on the atomic clock with an offset to compensate for the imprecise observed solar time. So TAI=UTC+UTC_OFFSET.
When the difference between the atomic time and the solar time moves by more than 0.9s, one second is added to the UTC_OFFSET, this is called the Leap second. The UTC_OFFSET is distributed by the GPS constellation to all GPS receivers.
Usually this Leap second is added the last day of December or the last day of June.
TimeLink equipments accomodate the leap second management and can per configuration either take it as it comes making a time jump or warn it has come without jumping in time so that a perfectly continous time is maintained.