Answer depends on different crawlers, but generally speaking:
While meta-tags used to play a role in ranking, this has been overused by many pages and therefore meta-tags are not considered by most search engines anymore.
Crawlers now mostly use meta-tags to compare keywords and description to the content of the page to give it a certain weight. For this reason while meta-tags do not play the big role it used to, it’s still important to include them.
Search engines index websites in order to respond to search queries with relevant information as quick as possible. For this reason, it stores information about indexed web pages, e.g. keyword, title or descriptions, in its database. This way search engines can quickly identify pages relevant to a search query.
Indexing has the additional purpose of giving a page a certain weight, as described in the search algorithms. This way search results can be ranked, after being indexed.
The process of making pages appear more prominently in search engine results is called SEO. There are many different techniques, considered in section C.2.11. This field is a big aspect of web marketing, as search engines do not disclose how exactly they work, making it hard for developers to perfectly optimise pages.
In order to check the web presence of a website, there are different metrics to be used.
Issues of parallel web crawling