SERP is an acronym for Search Engine Results Page.
A SERP 1 is better than a SERP 2 and a SERP 3 or worse really means your website needs some SEO (Search Engine Optimization) done for whatever the keywords that were used to bring up your website in the results.
A website has a SERP 1 for a particular set of keywords that are used in a Google search when that website appears in the first page of results. If a website appears on the 5th page of results, then that website has a SERP 5 for that particular set of keywords.
As an example, a plumbing company in Peoria Arizona is likely to get a poor SERP for the keywords
plumbing, water backed up
but could get a good SERP for the keywords
Peoria Arizona plumbing water backed up