SERP stands for Search Engine Results Position. If you type your keywords into google and your site comes up 15th - that is your SERP for those keywords.
I guess you are new in Search engine optimization, this is term for website ranking position over different search engines for the particular set of keywords.
When you operate a business online you need to let people know you exist and the best way for people to find your business is through the Search Engines, but without a good Search Engine Raink Position (SERP) your business will not be found.