直す originally means 'to fix' or 'to correct', but when attached to another verb's ます-stem, it adds the meaning of doing that action over again, usually to make it better or to fix a mistake. It's a common compound verb pattern in everyday Japanese.

This pattern is used when you are not satisfied with the first attempt and want to redo it. It implies a conscious effort to improve. For example, if you write a report and then realize it's full of mistakes, you would 書き直す (rewrite it).

You can use this with many action verbs, but not with verbs that describe states or involuntary actions. It's a practical, spoken pattern, though it also appears in writing.