Simplifying in math generally refers to fractions. To simplify a fraction, find the highest number that divides into both the numerator, or the top number, and the denominator, or the bottom number. Then, divide that number into both parts of the fraction to simplify.
The highest number that divides into both parts of the fraction is called the greatest common factor. If you are trying to simplify the fraction 8/12, you find that the greatest common factor is 4. Divide the numerator by 4 to get 2. Divide the denominator by 4 to get 2. Therefore, simplifying 8/12 leads you to the answer of 2/3.