Here we go. Far superior to the
British Gravitational or
English Engineering systems of units.
Basic units are length, force, and mass, which are interrelated by Newton's Second Law of Motion.
Length--meters (m).
Force--newtons (N). This is the derived one.
Mass--kilograms (kg). Actually, the gram is the base of the kilogram, but kilograms are used as the base of this system
So by the Law, one newton equals one kilogram*m/s^2, i.e. the force needed to accelerate a one kilogram mass at a rate of one meter/second/second.
Many people are familiar with the kilogram, even if they don't use it much, but erroneously think it is a measure of weight--likely because metric scales read in kilograms. As noted above, one kilogram will weigh 9.81 newtons on the earth, as the acceleration due to gravity here is 9.81 m/s^2. Take those scales to the moon, and what they say is one kilogram is really quite a bit more massive.