After a 2♦ response to the 2♣ enquiry, a further enquiry bid of 3♦ asks about three-card major suits. In reply, 3♥ shows three hearts while 3♠ shows three spades.
Some people play that opener has two further options, to bid 3NT with 2-2 in the majors or 4♣ with 3-3 in the majors.