I second the idea that the work should be centre stage. My current portfolio is just a nice render or photo on a white background with the client logo in a corner. I’ve never sent my portfolio away, so I stopped putting explanations as I’ll be there to explain it myself.
However, a good place to check on is this small book: Notes on Graphic Design and Visual Communication by Gregg Berryman. It’s the best text book I’ve ever used.
https://www.amazon.ca/Notes-Graphic-Design-Visual-Communication/dp/1560520442