An extra second will be added to 2016, meaning this year's December 31 will be a second longer than any other December 31s in the past years.
The "leap second," as it is scientifically called is not uncommon as extra seconds are added either at the stroke of midnight on June 30th or December 31st of any given year.
Why do we add seconds? These seconds are added depending on whether or not the adjustment is needed. It is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time. In simpler terms, to keep all our clocks in sync with the Earth's rotation.
As explained by Science Daily, historically, we have two time scales, one that is based on the mean rotation of the Earth relative to celestial bodies and the second was defined in this reference frame. However, in 1967, we switched over to UTC, which is based off atomic clocks that run with extreme precision and is independent of the Earth's rotation.
Although this may sound as if it is more accurate, problem rose since then. Vox mentioned that Earth's rotation has slowed, meaning days have generally been a bit longer. As a result, discrepancy has formed between solar time and official time. Thus, the extra seconds added.
Who monitors the discrepancy? The International Earth Rotation and Reference Systems Service (IERS) monitors the difference between the two time scales and they decide whether an extra second shall be added to the UTC. It is necessary to keep them within 0.9 seconds of each other. Since leap seconds started getting added in 1972, fully 27 out of 44 years have included one. The last was June 30, 2015.
When does it happen this year? When exactly that leap second will be added depends on your time zone. But as mentioned in IERS statement, "The sequence of dates of the UTC second markers will be: 2016 December 31 23h 59m 59s, 2016 December 31 23h 59m 60s, 2017 January 1, 0h 0m 0s."