A kilogram is the SI unit of mass and one of the 7 base units. It is the only SI unit with a prefix. The prefix "Kilo" is used for both the name and symbol of kilogram and is equal to 1000 or 103 grams. Originally the gram was going to be the base unit, but due to historical reasons regarding an artifact used to define the kilogram, the kilogram was chosen as the SI base unit for mass. As of 2018, the kilogram (kg) is now defined by taking the fixed numerical value of Planck's constant h to be 6.62607015 × 10−34 kg m2 s−1. This decision was made in order to ensure the long-term stability of the unit of mass. 
For a fascinating video on the origin of the kilogram and the impact of the new definition of a kilogram see Veritasium's video below.