A fraction is in simplest form if the top and bottom have no common factors other than 1. In other words, you cannot divide the top and bottom any further and have them still be whole numbers.
You might also hear simplest form called "lowest terms".
For example, the fraction is in simplest form. There's no number that divides both 4 and 5, other than 1.
The fraction is not in simplest form. You can divide both the numerator and denominator by 3.
Since you cannot divide 1 and 2 any further, you know that is the simplest form of .
