It can consist in the expansion of the belly, of the breast, butt, other parts or the whole body. The inflation usually is done with air (usually with a helium tank or an air pump) or with water, but this could happen for others reason (like a magic potion or a experimental medicine).
To better understand I highly recommend to see this awesome video of Cleverfoxman that explain in a clear and simply way what is the body inflation fetish.
Actually there are many types of body inflation, with a lot of possibile combinations, here you can see the most common types.