To add or subtract you must first find a common denominator (the number below the line). One way is to multiply them, but is more common to find the smallest. You can multiply (or divide) both numerator and denominator of a fraction by anything other than zero and its value will not change. So the idea is to get the denominators the same and just add the numerators. For example 1/4 + 1/3, smallest common denominator is 12, so multiply 1/4 by 3(top AND bottom), to get 3/12, and 1/3 by 4 (top AND bottom) to get 4/12 add them to get 7/12