A proportion is a relative portion of a whole, as opposed to a count or frequency. For example, suppose your company manufactures a silver alloy for photographic film. If you were interested in the proportion of silver (by weight) in the mixture, you aren't worried about how many pounds of silver your plant uses in a day, you're worried about the portion of silver relative to the entire alloy. Assuming that precisely half of the alloy is silver, the proportion of silver could be expressed as a percentage (50%), a decimal (0.5), or a fraction (1/2).
Proportions let you compare groups of unequal size. For example, a call center that handles 10,000 calls a day might record many more dropped calls than a facility that handles only 500 calls, but they may both have the same proportion of dropped calls.