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 [ᴬ]. Artists such as [a916418] 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.
[ᴬ] Generally, it is recommended to use context and common sense regarding this statement (not being too formal). For example, if artist 'N' records his live musical performance obviously disguised as artist 'X' and then promotes is as a field recording of something one himself was not involved in, it might seem appropriate to still not tag it as field recording, because we just don't pose live performances as field recordings per se. The example with a recorder and their child is quite liminal, implying that a child is not conscious about making music.