SERP stands for search engine results page: everything Google shows after a search. Besides the classic blue links, it can include ads, an AI Overview, a featured snippet, "People also ask" questions, videos, images, maps and shopping results.
Why it matters
The SERP decides how much attention each result gets. Ranking third on a page that opens with ads and an AI Overview can bring fewer clicks than ranking third on a plain page.
Example
A search for "how long to steep cold brew" may show a featured snippet with the answer, a "People also ask" box and a video, all above the regular results.
What to do
Look at the SERP before you write. Its features tell you what Google thinks searchers want, and which formats (a quick answer, a list, a video) give you the best chance of showing up.