What is a domain?
A domain is the address people type to reach your site, like example.com. You rent it, usually a year at a time, from a registrar. It is separate from the site itself: the domain is the address, the hosting is the building it points at.
The important part is whose name it is registered in. A domain registered to you can be pointed anywhere, moved to any provider and taken with you if you change tools. A domain somebody else registered on your behalf is theirs, and getting it back can be slow or impossible.
If you take one piece of advice from this glossary, take that one. Register your own domain, in your own name, with your own card. Everything else in a project can be rebuilt; an address people have already learned cannot.
Do I need a domain to put something online?
No. Most tools give you a free address on a subdomain of theirs, which works perfectly well and is a sensible way to start. You want your own domain when the thing becomes real, or when you need it to look like it belongs to you rather than to a platform.