Sound files can be stereo, with a right channel and a left channel, or they can be mono with just one channel. Here's what a stereo file looks like:
Here's what a mono file looks like:
Edit operations like cut, copy, and paste affect both channels on a stereo file. When you add effects from the Effects menu like EQ, Bass Boost, and Echo you have a choice between adding the effect on one channel or both.
You can convert a stereo file to mono or a mono file to stereo by clicking Edit->Change Format.
Note: If you convert stereo to mono, it will mix both channels together; once this is done, you cannot get the channel separation back by attempting to convert it back to a stereo file.