What?
Whatever we see in the lightning experience (and sometimes in Classic too! Yes, truly) is essentially built out of a lightning component.

One way to put it, a lightning component is the building block or smallest self-sufficient unit that helps construct the lightning experience. It can be as big as an entire lightning page or as small as the spinner. Nah, not the one the kids play around with these days. The lightning spinner. The circle of dancing dots that appears whenever you do something and the expected outcome doesn’t appear instantly. It’s the salesforce way of saying, you gotta wait son! whatever you need, isn’t ready yet! And the lightning spinner just lets you know that the system is working really hard to serve your request. Now the lightning spinner is something that salesforce provides us out of the box. We don’t need to worry about building it. But if you want something custom made, then you need to get to work and build your own lightning component.