It is a google search operator which allows searching a result from a specific website. This search operator restricts your results to a single website. There is no space between the domain name and the site operator.
Example: site:abc.com
If you want to quickly search a website's content on a particular topic, use a search word after the site command like this:
Example: site: abc.com visa world
This looks for all of the articles on the ABC website that discuss using a Visa World Card.
When used alone, you may do a generic search and quickly check if the pages in your database and those that are indexed match.
You can use these operators to browse certain websites or to focus your search in ways that are unnecessary for most ordinary people.
However, Google emphasizes that this should not be used as an audit. It restricts the results to a certain website, albeit it may not always return all of the pages from that site that are relevant to the search. That has a little more important for site owners who are seeking the indexed pages of their own websites.