When the User click keywords or hyper link it will not be reach the particular landing page. It will be called "404 Not found Page" error.

Blunder 404 is a standout amongst the most widely recognized web mistakes we find in our everyday Internet surfing. We realize that mistake pages offer a clearly negative client encounter.

404 is Page not found error that means that error appear when page is not available in website.

The HTTP 404, 404 Not Found and 404 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 HTTP 404 Not Found Error implies that the site page you were attempting to reach couldn't be found on the server. It is a Client-side Error which implies that either the page has been expelled or moved and the URL was not changed likewise, or that you composed in the URL inaccurately.