Sikter is right. I'll expand on that a little more for clarity.
If you set the track input source to be Stereo 1-2 (for example) then the track expects to receive two inputs, (one from channel 1 and one from channel 2.) The software then records the two channels side-by-side on the same track and outputs them to separate main channels as well, (L and R.)
If you set the track input source to Mono 1 the track expects a single input (from input 1,) and during playback sends the same identical signal out through both L and R main channels.
In stereo mode, having only one mic plugged in fulfills the left side but not the right.
Hope this helps,
RD