Direct Answer
Quality score is a 1–10 rating Google assigns to each ad keyword based on expected click-through rate, ad relevance, and landing-page experience. Higher quality scores lower your cost-per-click and improve ad position. Aim for 7+ on all primary keywords; below 5 is a sign the landing page or ad copy needs work.
Voice answer (≤30 words)
“Quality score is Google's one to ten rating of your keyword based on click-through rate, ad relevance, and landing page experience.”
Quality score is the cheapest CPC reduction lever. A quality score of 8 vs 5 can cut CPC 30–50%.
Match landing-page H1 to the ad's primary keyword exactly — single biggest quality-score lift in most accounts.