Literally, when we thought it was all over, it wasn't. There were more or unknown things waiting for us.
This expression can be used in various situations, such as a series of problems or challenges in work, study or life. When we thought we had solved everything, new problems or challenges appeared again. Or, we may face an endless state, such as constant anxiety, worry or uncertainty about the future, which makes us feel unable to move forward or end.
Generally speaking, this sentence expresses a general sense of uncertainty about life and emotion and a sense of confusion about the future.