There are numerous variations, but the most common is
(a) 2D=suits of same rank (both majors or both minors
(b) 2H=suits of same color
(c) 2S=alternate suits (clubs and hearts or diamonds and spades)
Opener has 5-10 HCP.
Response: Start bidding up the line to find a fit. (E.g., opener bids 2D, responder bids best major; if opener rebids a minor, s/he is showing both minors, and responder chooses better minor).