We have been concerned up until now with the opener and their partner the responder. You are not forbidden from bidding just because you aren't either of these. If you bid after your opponents have opened the bidding, you are now called an overcaller .
As an overcaller the rules of bidding change and are now quite different and instead of trying to get to game, you are now attempting to:
In the whole card deck there are only 40 high card points (HCP) altogether so If the opener and the responder have been bidding this means it is unlikely (not impossible however) that you and your partner will have enough HCP to bid to game.
If you have a good 5-card suit you can simply bid that suit even with less than 12 TP. All you promise is a good suit (remember a good suit is 2 of the top 4 honours) so partner shouldn’t expect a lot of points.
Bridge Instruction Online - Acol Lesson 8