A trencher is, simply, the bottom half of a loaf of bread which is used as a plate. The loaf is cut horizontally lengthwise, then food is piled on and consumed. In period, trenchers themselves were not eaten, but thrown to the dogs (or occasionally the poor). If one was really wealthy, they placed the trencher on a plate, rather than on the table itself.
Bread trenchers were replaced by wooden trenchers (possibly for reasons of economy) which were generally square with a shallow dishing in the top. Later trenchers also have a secondary smaller dish, possibly for the containment of salt after it had been removed from a salt cellar.
For those who wish to try eating from a medieval trencher, simply cut a loaf of thick, coarse bread (such as pumpernickel) horizontally along its length. Set the top portion aside, and use the flat-bottomed half as a plate. Pile medieval foods on top of it and dig in. Do not try this on your lap, (or on mum's good linen tablecloth) as a certain amount of gravy-leakage can be expected to occur.