Hi! The input-output relationship is usually a measure of the efficacy of transmitter release. The "input" is measured from the size of the pre-synaptic fiber volley, while the "output" is usually the slope of your field EPSP. In acute slices, it is often used as an indirect measure of how viable the tissue is.

It involves both pre- and post-synaptic mechanisms, because it is a ratio between both. The size of the fiber volley is related to the number of fibers you are stimulating, i.e. pre-synapses. On the other hand, the size of the fEPSP depends on how much neurotransmitter is released, how many receptors are bound by it, and how efficient the signalling mechanism is on the post-synaptic side.

By itself it does not really tell much. But it is useful when you compare two conditions, for example tissue from normal or epileptic animals. You can quantify the size of fEPSPs at the same "input" levels (or fiber volley size) and see if your pathological conditions make any difference.

Hope this helps!