It would be helpful if you were to precede the report with an introduction.
The sentence is correct.
The meaning of the sentence is that it would help if the report-writer added an introduction to the beginning of the report. (So the introduction would precede the actual report.)
As mentioned by @murat in the comments, this other post is of interest.
Essentially, this is the subjunctive mood, which is used primarily for hypothetical situations. From the linked Wikipedia article:
typically used to express various states of unreality such as wish, emotion, possibility, judgment, opinion, necessity, or action that has not yet occurred.