Add and subtract fractions with the same denominator and denominators that are multiples of the same number

What this means, in plain words

The denominator is the number underneath the line in a fraction. It tells you how many equal parts the whole was split into.

A multiple of a number is what you get when you multiply it by a whole number, so 12 is a multiple of 3.

A fraction describes part of a whole, or part of a set, once the whole has been split into equal pieces.