Using your own breath or an air pump, you can inflate a balloon — or anything else you need to pump air into. When you inflate something you fill it with air (or any other gas) to make it expand.
From the Latin verb inflāre, "to blow into," the word inflate appeared in English in the early 16th century. Although we usually think of the word inflate as referring to blowing air or gas into a balloon or a tire, anything that grows larger, from food prices to an egoist's self-image, can be inflated. When an item becomes scarce, its price is likely to be inflated.