A fraction is in its simplest form (this is also called being expressed in lowest terms) if the Greatest Common Factor (GCF), also called the Greatest Common Divisor (GCD), of the numerator and denominator, is 1. For example, 1/2 is in lowest terms but 2/4 is not.
To reduce a fraction to lowest terms (also called its simplest form), divide both the numerator and denominator by the GCD. For example, 2/3 is in lowest form, but 4/6 is not in lowest form (the GCD of 4 and 6 is 2) and 4/6 can be expressed as 2/3.