SERP stands for Search Engine Results Page, and is the shortned name of the pages where the result are displayed to users.
Some search engines also display advertising on their result pages, in addition to the normal results. These are however usually easy to tell apart, from the search results them self.
The term SERP might be most frequently used in SEO circles, when talking about PageRank, Backlinks, and other such topics of SEO.
The below example shows how beamtics SEO Tutorials has looked on a SERP of Google in the past:
The problem with the SERPs
Since most users generally won't click on links placed further down in the results lists, this creates a real problem within todays search engines.
Rotating the top results could remove some of the pressure, but would likely annoy end-users.
Generally well-established sites rank higher, but newer sites, with equally good content, may not rank at all, which is a real problem for those websites. The end-users likely don't care, as long as the existing sites are doing it well enough.