A GBA file is a ROM image of a Nintendo Game Boy Advance (GBA) video game, such as Pokémon, Zelda, or Final Fantasy. It contains an exact copy of a GBA game saved in a format that can be played using a Game Boy emulator.
You most likely will only see a GBA file if you are attempting to play GBA games on a computer or mobile device. Since the games were not meant to be played on devices other than the GBA, they must be opened by an emulator in order to play them.
There are a variety of emulators for Windows, macOS, Linux, Android, and iOS. Some of the emulators are NO$GBA, VisualBoyAdvance, DreamGBA, KiGB, GBA4iOS, and GBAoid.