What is a Filler?

. . . a material that is used to fill in an area that is depressed, has lost volume, or considered a "wrinkle". Fillers can be naturally occurring substances (like fat) or synthetic. Before many of the fillers on the market today were available, some of these materials were surgically placed under the skin, and in thes cases, would be termed an "implant". Both implants and fillers are available today, and each have their own role in aesthetic surgery.

Fillers are various substances, both natural and synthetic, that are injected to fill depressions.  Wrinkles are depressions and fillers find their main use in this area.  They are also used for acne scars, depressed traumatic scars, etc.