on a mixer, the insert is a jack that has both input and output at the same time. it uses a stereo cable and either the input our output is on the Left side and the other is on the Right (forget which is which). they are designed so that if you put a 1/4" cable in half-way, you get the out, but the signal keeps passing through the mixer channel; it doesn't look for the "in" signal. just acts as a direct out. the insert on your compressor is for a sidechain. you usually patch an eq in there. the eq does not effect the signal, but rather the signal that the compressor acts on. so if you cut all the lows on the eq, they wont affect the compression threshold. you can use this for "ducking" and other techniques that you'll probably never use.