Stereo to monoMixes the left and right channels into one. Mono files are about half the size, and are what many games, apps, voice and phone systems expect.
Mono to stereoCopies the sound to both sides. It does not create a stereo effect, but some programs need two channels.
Sample rateHow many times per second the sound is measured. 44,100 Hz and 48,000 Hz are standard for music and video; 22,050 Hz and 16,000 Hz are enough for speech; 8,000 Hz is telephone quality. Lowering it makes the file smaller but the sound duller, so press Play result before you download.
MP3 only supports some sample rates (from 8,000 to 48,000 Hz). If you choose MP3 and your setting is not supported, the closest valid one is used and shown in the result line.