Upon looking under the list of posts I flagged, some are pending/ helpful. Under those helpful ones, some are highlighted in red. What does that mean?
If by "highlighted in red" you mean a pink background behind the question title,
then that indicates that the post in question has been deleted.
If you mean something else, then you should provide a screenshot and be more descriptive about the circumstances (type of flag raised, whether it was on a question/answer/comment, etc.).