Reverse Bids

When opener bids a second suit at the 2-level in a higher ranking suit than the opening bid it's known as a reverse.

Opener will normally try to show another suit on the second round of bidding rather than bidding the same suit twice.

North is happy in clubs.

North prefers diamonds.

A reverse means that responder has to bid to the 3 level in order to return to opener's first suit.
A reverse means that responder has to bid to the 3 level in order to return to opener's first suit.

If North wants to go back to South's first suit it will need to be at the 3 level with a bid of 3♦.

2♣ is not a reverse.

North can bid 2♦, returning to opener's suit at the 2 level.
If you're not strong enough to reverse, make the lowest bid you can that shows something extra about your hand.

2♦ shows 5+ diamonds

Some players have the agreement that a reverse is forcing for 1 round, others have the agreement that a reverse is forcing to game.
On the following hand North has reversed showing 5 clubs, 4 diamonds and a strong hand.

You have a club fit.
This time you are stronger and it's up to you to indicate your extra strength.

You have spades covered, bid 3NT.

North bids 1♥, the lower ranking 4 card suit. South bids 1♠ which is not a reverse because it's a bid at the 1-level.

North bids 1♠, his longest suit. South is not strong enough to bid 2♥ which would be a reverse so has to settle for rebidding diamonds. North now bids 2♥ showing 54 shape and, again, the trump fit is found.

South rebids 1NT to show a balanced hand. There's no need to bid hearts! If North has hearts he would have either bid them on the first round or else he'll bid them next round.