This means to simplify the fraction as much as possible by using greatest common factors.

For example:

4/8 is not expressed in lowest terms because 4 goes into both 4 and 8. So you just divide the numerator (top) and denominator (bottom) both by 4 and you are left with 1/2.

For example:

4/8 is not expressed in lowest terms because 4 goes into both 4 and 8. So you just divide the numerator (top) and denominator (bottom) both by 4 and you are left with 1/2.