Answer depends on different crawlers, but generally speaking:

• The title tag, not strictly a meta-tag, is what is shown in the results, through the indexer
• The description meta-tag provides the indexer with a short description of the page
• The keywords meta-tag provides…well keywords about your page

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.