SERP stands for "search engine results page": everything Google shows for a single query. That is more than ten blue links — it is the organic results, the local pack (the map and its businesses), ads, People-also-ask boxes, featured snippets, and any other panels Google assembles.
The key idea for local SEO is that there is no one SERP for a keyword. The same query produces a different page in every city and country, because Google localizes and personalizes what it shows. Comparing two SERPs side by side is how you see that difference instead of guessing at it.
See how SERP changes real results: compare Google from any two locations.
Deeper read: Why your keyword has no single Google ranking