Websites are hosted on servers somewhere on the internet. The company that owns such servers rents them out to mutliple web sites. It is an economical use of space since the cost of the server is shared by several web sites. Usually this is unnoticeable to any of the web site users but sometimes one of the web sites can make excessive demands on the server whereupon all of the sites on that same serve can be slowed down. There can be other issues if other web sites send a lot of spam too.
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