What is SERP?

SERP stands for Search Engine Results Page. It is a ranking of a website on a search page according to the search results for a particular keyword searched.

Like for example:

You searched for keyword “JavatPoint”

Here for this keyword the website :
www.javatutorialpoint.com appears to be at rank 9 

while the website 
www.javatpoint.com appears to be at position 1.

So this is search engine results page for a particular keyword searched.

Note that, for the same keyword the ranking of a particular website may vary from cities and countries where it is searched. Like for example here the website www.javatutorialpoint.com is rank 9 when searched from Delhi, while it could be some other in usa, uk, etc. 

Also the rank of a website for this keyword is at rank 9, while it could be different for other keywords.

They better the Rank (1-3), the more likely a user visits that website.