The second is the SI base unit for time. It is defined by 9,192,631,770 oscillations of the radiation corresponding to the transition between two levels of the caesium atom.
The second is used to measure time. As well as enabling us to tell the time of the day, accurate timekeeping underpins the functioning of satellite technology like GPS or the internet and facilitates timestamping for transactions in financial trading.
Ancient civilisations used sundials and obelisks to tell the time, which was not very precise, and was restricted in cloudy weather or at night. Time was commonly measured in terms of days, relying on the rotation of the Earth. This rotation is not regular enough to give this definition sufficient accuracy.
The second is not expected to be redefined in 2019.