A proform is any word which can replace another word, phrase or clause in a sentence or utterance when the meaning is recoverable from the linguistic or extra-linguistic context. Proforms thus avoid unnecessary repetion or redundancy.
The most common types are pronouns, which replace nouns or noun phrases.
John isn't here today. He's ill (He replaces John.)
I need a new t-shirt. I'll buy one when I go to the market on Friday. (One replaces a new T-shirt).
That's the man who told me (Who replaces the man.)
However, other words can also be proforms. For example, does acting as operator can replace a full verb phrase :
I don't like dogs, but Jane does (Does replaces the verb phrase likes dogs)
I was at the market this morning and I bought a t-shirt there. (The adverb there replaces the prepositional phrase at the market)
I'm not sure if John will be at the meeting, but I think so. (The adverb so replaces the clause that John will be at the meeting).
Proforms are thus devices of reference and substitution.