Hi; I can't remember which way round the stereo plug is wired in terms of the tip and the ring, but most insert cables (good ones anyway) come with the two mono plugs labelled as 'in' and 'out' or 'send' and 'return'. These cables are used when you want to use an external effects machine (eg a compressor, Eq or reverb)on a single channel, subgroup or main mix on the mixer. Plug the stereo end into the appropriate insert hole on the mixer, the 'send' plug into the input of the effects box and the 'return' plug into the output of the effects box. Set up the effects box as you want it, and then you can usually mix the amount of FX you want in the mix at the mixer end.
Tim