The study of Physics is based on experiments, which allowed us to test and validate theories. The results of those experiments are obtained by accurate measurements using physical quantities.
All physical quantities consist of a numerical magnitude and a unit.
There are seven base quantities (E.g. Mass, length). From the seven base quantities, you can obtain all the other physical quantities.
Physical quantities had many different types of units in the past. In the modern times, the units have been standardised and are named the SI units, from the French “Le Systeme International d’Unites”.