Units of measurement are the terms that we use to describe the size of something. You probably know that units include things like seconds, kilograms, meters, and so forth.
Without standard units of measurement, scientists would have a huge problem understanding what other scientists were saying. After all, if I were to refer to a meter as "the length of my leg" and another scientists were to refer to it as "the length of a Saint Bernard dog", we'd have a lot of trouble when it came to do actual science. To avoid this problem, the SI system of units gives us a convenient and, more importantly, standard set of units that we can agree on.
