I'm not sure exactly what it means in context of gmail, but in the broader sense of domains, it's something like:

Say you own example.com. You want to have a main website, and you decide to put it at www.example.com. www is the subdomain that's set up for the website.

Say you want to have a different website on a completely different server. The easiest way is to set up a separate subdomain, something like website2.example.com. website2 would be the subdomain.