Field recording is a style of non-music. Field recordings are audio recordings of sounds, often non-musical, but music can be recorded as well. Artists of this style typically have zero involvement with the sound of the piece, they are simply recording "the field". Field recordings can be of nature (bird songs, rain, the ocean, etc), of locations (markets, factories, restaurants, etc), of events (parades, family gatherings, etc), and countless other subjects.
Field recordings can also include recordings of electrical systems, space noise, and so on. Field recordings can also be musical if the subject is of music; but only when the field recorder is recording music that they themselves are not involved in. Artists such as [a=Kink Gong] are well known for their musical field recordings. If the field recorder is involved in the music they are recording, it is no longer a field recording, but special exceptions do occur (such as an artist field recording themselves playing with toy instruments with their child).
An alternate name of field recording is phonography, the sound equivalent of photography. In the case of field recordings, typically it is of things the artist is not directly involved in. Or if the artist is involved it is naturally occurring sounds such as movement, talking, eating, and so on. The concept of field recording being "sound photography" should help you know when to apply this style tag and when to not.