what is 404 error in linux ?

404 error is a normal error  that the webpage/URL  you were trying to reach could not be found on the server. It is a Client-side error  which means that either the page has been removed or moved and the URL was not changed accordingly, or that you typed in the URL incorrectly.

for example below webpage has been removed and no longer exist. 

if your trying to access the above webpage you will get 404 error .

same kind of 404 eror in linux you will get when your trying to access http url and yum repository .

lets say you get this error when your trying to run any "yum" command it means the repo URL which setup in your repository is no longer exists .