Google has computers that search the Internet looking for new web pages. This is called crawling.
When Google finds a page it decides whether it is any good or not. If it decides it is useful it will add it to it's search result pages.
This is called being indexed, being added to the list of pages that can be found during a search.
If later Google does not want to list your page they can "deindex" it and remove it from all search result pages.
This is called being dropped.