When thinking about numbers, a unit is some agreed standard measure of some physical quantity such as weight, or length.
For example, one such unit is the kilogram, which is a unit of weight (abbreviated kg). So we know that something with a weight of one kilogram ( 1kg) is exactly half the weight as another object with a weight of two kilograms (2kg)
These units are agreed upon internationally by various "Standards" organizations. For example the kilogram is defined by the General Conference on Weights and Measures (CGPM) which meets every four years near Paris.
It was originally defined as the weight of a liter of water, but now has a much more accurate (and complex) definition.