The 404 Error Page, known as "page not found," is an HTTP[S] standardized status code that indicates an established connection with the server but an inability to find the page in question.

404 error tells us the requested page is not available in server

The protocol 404, 404 Not Found and 404 (pronounced "four Buckeye State four") error message may be a machine-readable text Transfer Protocol (HTTP) commonplace response code, in electronic network communications, to point that the shopper was ready to communicate with a given server, however the server couldn't notice what was requested.

404 Error
 

There are different type of 404 Errors. Like

404 Not found Page
404 Soft Error
404 Server Error

It means the website is not found .......................

404 Error means that the page you were looking for was not found. Usually, it is for two reasons:

You inserted a wrong URL
The page has been removed

Error 404 means that the server could not find the web page...it is called also HTTP standard response code,the meaning of that is server unable to connect with user..

The 404 Not Found Error is a HTTP reaction status code, which demonstrates that the asked for asset couldn't be found. Like most HTTP reaction codes, and especially for those codes that show an error, the reason for a 404 Not Found Error can be hard to find and resolve.

When a page disappears from the website and Google also unable crawl the pages on a website.

The HTTP 404, 404 Not Found and 404 (pronounced "four oh four") error message is a Hypertext Transfer Protocol (HTTP) standard response code, in computer network communications, to indicate that the client was able to communicate with a given server, but the server could not find what was requested.

The 404 Not Found Error is a HTTP reaction status code, which demonstrates that the asked for asset couldn't be found. Like most HTTP reaction codes, and especially for those codes that show a mistake, the reason for a 404 Not Found Error can be hard to find and resolve.