As the name suggests, a subdomain is a subdivision of a domain and exists beneath it. For example, "my.unlimitedhosting.me" ('my' is a subdomain of the unlimitedhosting.me domain). You can use it if you want to highlight something special. For instance, instead of "mojadomena.com/forum/", you can create "forum.mojadomena.com", which looks nicer and is easier to remember.