✔ 最佳答案
For instance if your english teacher has asked a question to your class and one of your classmates has given her an answer, when she then replies 'that will do', it means thhat answer is fine, but not what she is thinking.
In another case where you are handing in a piece of work to her but for example have forgotten to bind those loosened pages together, if she is nice enough, she may say 'don't worry, that will do (too)', which means that is okay (yet not ideal, as she may then remind you to bind the pages together next time after saying 'that will do'.
In general it means 'that's fine', though not excellent.
Hope it helps.