A stereo file holds two channels, and most channel problems are routing problems: the wrong side is empty, the pair is reversed, or the file needs to be one channel instead of two.
Sum to mono
Sum to mono averages left and right and puts the result on both channels. Averaging rather than discarding a side keeps anything panned hard left or hard right in the output.
The average is also a test. Content that is identical but opposite in the two channels cancels here, so a mix that thins out or loses its bass in mono has a polarity problem somewhere upstream. Voice recordings from a two-microphone setup are the usual offenders.
Ticking the single-channel output writes an actual mono file rather than a stereo file carrying the same signal twice, which halves the size of a WAV or FLAC export and takes a real bite out of a lossy one.
Left, right, and swap
Swap left and right is the fix for a reversed pair, whether that came from a cable, a patch, or an interface with its inputs crossed.
Left to both and Right to both copy one channel over the other. Use them when only one side of a recording is usable: an interview where the second microphone was never plugged in, or a capture where one channel clipped.
Side only
Side only keeps the difference between the channels, (L - R) / 2. Anything sitting equally in both channels cancels, which strips most centred lead vocals, bass, and kick from a mix.
Two limits are worth knowing before reaching for it. Anything panned off centre stays, so guitars, keys, and stereo reverb survive intact. And a mono source has no difference to extract, so the result is silence.
Phase inversion
Inverting flips the sign of every sample in a channel. On its own, through speakers, an inverted stereo pair sounds the same as the original, because hearing is insensitive to absolute polarity.
It matters as soon as two signals meet. Inverting one channel of a pair that already agrees will cancel the shared content on the way to mono, and inverting a channel that disagrees will fix it. Flipping both changes nothing audible.
Channels past the first two
A file with more than two channels keeps the extra ones untouched. The routing acts on the front left and front right pair, so a surround centre or LFE track passes through unchanged.