Each bid must be higher-ranked than the current highest bid.   If not then it is termed "insufficient".

Acceptance

If a player makes an Insufficient Bid then the next player has the option of accepting this bid.

If the next call is made before anyone has noticed, then the illegal call has been accepted, you cannot go back. 

Correction

If the next player doesn't accept the Insufficient Bid then the offender must change it to a legal sufficient bid, but they may not Double.

They can make a bid that shows the same denomination at the lowest sufficient level   i.e. same suit

They can make a comparable call, a bid that has a similar meaning, same purpose, or is a subset of the insufficient bid.

They don't have to make one of these bids, but if they don't then their partner is barred from the rest of the auction, and there could be lead restrictions if they become defenders. 

Example

North opens 1S and East bids 1H.

Firstly, South may accept the 1H bid and continue the auction, maybe bidding 1S or even passing.

If not, then the 1H bid is withdrawn and East has to make a sufficient bid, but cannot double.

If East bids 2H then everything is fine, the auction continues with no more issues.

East can make any other sufficient bid - 1NT or 2D or 4H if they like - but in this case West will have to pass for the rest of the auction.   And if East-West become defenders then the first time West is on lead, Declarer can prohibit the lead of any suit not bid by East after the insufficient bid.