A fraction is in simplest form if the greatest common factor (GCF) of its numerator and denominator is 1.
For example,

is in simplest form, since 3 and 11 have no common factors other than 1. However,
is not in simplest form, since 6 and 21 share a common factor of 3.
To reduce a fraction, divide both the numerator and denominator by the GCF.
This is sometimes shown as "canceling" the common factors.
Note that the result is an equivalent fraction in simplest form.