De-indexing refers to a website or a being removed from Google’s (or other search engines) index. Deindexing happens if the content is low quality or spammy, if the site is down for an extended period or if the site owner requests to remove it.
When a site is de-indexed, it no longer shows up in search results. Though being de-indexed is a major setback, it is not permanent. The site can be re-indexed once its content has been improved or updated.