Answer:
Suppose you are in a cinema and there are 12 men and 36 women, then the ratio men to women is 12:36, dividing each term by 12 we get 1:3, which is an equivalent ratio. Other way to get an equivalent ratio is multiplying each term by some constant, for example if we multiply each term in the ratio 1:3 by 4 we get the equivalent ratio: 4:12.
Ratios are used to show the relative sizes of two values, in the example, the relative amount of men and women in some place.