A bracer is a piece of archery equipment that protects the inside of one's forearm when using a bow from 'string slap' (where the released string strikes the forearm as the bow straightens).
Bracers are generally made of leather, although examples made of horn and ivory have been found.
They differ from vambraces in that they are generally much smaller, have the buckles or lacing on the outer part of the arm and were worn singly.
Medieval bracers are quite small and it is thought that there may have been some amount of 'see how tough I am' associated with using a small bracer. It could equally have been economy.
Historically, the English also used the term to describe plate defences for the entire arm.