Mass is defined as the amount of matter within a substance or object. Mass differs from weight, which is a measurement of the force of gravity on a substance.
The standard metric (or International System of Units, abbreviated as SI units) unit of mass is the gram, or any conversion of it (such as milligram, centigram, nanogram, decigram, kilogram, etc). One gram is approximately the mass of one paperclip.
It is important to have standard units in science. Standard units allow for a common language of measurement between scientists around the world.
Just as the gram is the standard unit of measurement for mass, there are also standard units for length (the meter), volume (the liter), time (seconds), and temperature (Kelvin). The following link explains standard base units and derived units more in detail.