A backraise is a reraise following a call.
Player A bets
Player B calls
Player C raises
Player A calls
Player B raises -> he makes a backraise, since he called the first raise and then reraised C's raise.
A special form of the backraise is the limp-raise. In games with blinds, calling the blinds is called limping. Hence, limping and then reraising a following raise is called a limp-raise.