What is a leap second?
Similar to a leap day, a leap second is an extra second added to the end of the day. Also like a leap day, the leap second serves to keep our measurement of time in sync with the Earth’s rotation. Unlike leap days, the occurrence of a leap second is variable and are announced only six months in advance because of the Earth’s irregular rotation.
What will happen when the leap second occurs?
What you will see depends on how your timepiece handles leap seconds. Those that have proper display support for leap seconds should display 23:59:60 UTC. Those that support leap seconds but cannot display them may show 23:59:59 during the leap second. Devices that do not support leap seconds will continue to 0:00:00 and will have to be manually adjusted.
No, not really. The passing of the leap second is no different from any other second and time does not stop for the duration of the leap second. The only reasonable concern is adjusting all your devices that do not support leap seconds.