After opener bids one of a major and opponents make an unusual notrump overcall, responder may make the following bids:
Double - you are interested in doubling at least one of opponents' minor suits. A subsequent double is for penalty, a subsequent pass shows not only that you're not interested in penalty, but have length in the other suit.
Cue bid the lower of the two suits shown (usually clubs) - this shows a limit raise or better in partner's major
Cue bid the higher of the two suits - this shows limit raise values or better and length in the fourth 'unbid' suit. There are dififerent agreements as to whether this shows 4 or 5, but four appears to be more common.
Bid partner's major at the 3 level - this is a competitive raise and semi-preemptive.
Bid partner's major at the 4 level - preemptive raise. With invitational or better you would've made a cue bid.
Bid 3NT - to play. This shows game going values AND shows a stopper in at least one of the suit. Some partnerships require stoppers on both suits.
Pass - weak hand with less than invitational or game going values.