Shakespeare introduces the concept of masks at the onset of the play. What information does this give us about the themes of the play?
Yes, there's a whole bucket of references and themes to the idea that appearance can be different to reality. In just the way that a mask makes the person seem not like the person, so too do all sorts of things in the play seem other to that they are.
I've just looked in the first scene of the play. Immediately, Claudio, the messenger tells us,
hath borne himself beyond the promise of
his age, doing in the figure of a lamb the feats of a lion.
He acts like a lion, but looks like a lamb. Quite interesting, if you consider what Claudio later goes on to do to Hero! And what is Benedick, the messenger is asked?
A lord to a lord, a man to a man; stuffed with all
It is so indeed. He is no less than a stuffed man...
Benedick looks like a lord if a lord looks at him, like a man if a man looks at him. Everyone wants to be like him, he looks great, the messenger says. But Beatrice thinks Benedick is only appearance - an outward skin, stuffed. Benedick is indeed obsessed with appearance:
Very easily possible. He wears his faith but as the
fashion of his hat; it ever changes with the next block.
Benedick changes hat and changes faith. The mask changes the face. It's one of the key themes of the play - think of the gullings, or the "non-adultery" which looks like adultery at the centre of the play. Or the way that Don John seems to be offering help, but is really causing trouble. The list goes on...