Two fractions are equivalent if they represent the same value.
For example, the figure below shows that and are all equivalent fractions -- in each figure, the shaded portion represents half of the whole circle.
In general, if you multiply the numerator and denominator of a fraction by the same (nonzero) number, you get a fraction equivalent to the one you started with. This is because when you multiply by a fraction like or , you're really just multiplying by -- and multiplying by always gives you the same number you started with. (Even if it looks different.)