This is the act of passing off as your own the work of others. It means copying without acknowledging that you have copied. It means losing marks, and even, possibly, having to re-sit an exam all over again.
Some exam boards now employ whole departments to check for plagiarism, utilising sophisticated software and search techniques that can identify whether writing is original or not.
If you are worried that you might have copied another's work accidentally, ask your teacher for advice.