In
medieval times, a trencher, although sometimes made of wood or metal, would most commonly have been a slice of thick, dark bread. In large households, bread for trenchers was bakes specially, from a coarse mix of
barley, oats,
rye and even pulses - wheat, being the most expensive
grain, was never used.
The trencher bread would have been a rather flat round loaf, which would be cut in half lengthwise and used as a food plate. The juices and sauces from the various dishes would soak into the bread, making it a popular day-after breakfast snack (often additionally soaked in broth or milk). It was also common to collect the used trenchers and give them as alms to the poor together with the other leftovers from a meal.
A piece of English trivia: the expression "upper crust", denoting priviledged or wealthy persons, comes from the fact that the lower halves of the trencher bread, which have been resting on the sooty bottom of the bread oven, would have been given to the lower status people in a household, whereas the upper, clean crusts would have been reserved for the use of the owners of the house and their higher status guests.