A responsive search ad (RSA) is the standard Google Search ad format, where you supply up to 15 headlines and 4 descriptions and Google assembles and tests combinations. It is now effectively the only Search text ad type.
How RSAs work
You provide the raw material, headlines and descriptions, and Google mixes them per auction to fit the query and the user, learning which combinations perform. Ad Strength gives feedback on how much variety and relevance you have supplied, though it is guidance, not a ranking factor.
Getting the most from them
Give genuinely distinct headlines rather than near-duplicates, include your main keyword in a few, and pin sparingly, since over-pinning removes the testing that makes RSAs work. Strong, varied assets feed the same Quality Score signals that lower your cost per click.