When the Mac was created back in the 1980's, Apple created a graphically interface for it using a "desktop" metaphor. The disk (storage volume like on a hard drive) is the "filing cabinet." In the disk are "folders." They are technically called directories, but "folder" fits the metaphor (so the icon looks like a folder). In the folders are documents of various types. The metaphor is not perfect; on the Mac, you can put folders inside folders inside folder... Out in the real world, most people do not put a folder inside a folder. But the purpose was to make users feel more comfortable with using computers for the first time, back in the "early days" of Mac. So the screen is a desktop, with storage units that have folders inside for storing documents.
A Finder window is a view into what you have stored. Open a Finder window, and click something like Documents in the sidebar. From the menu bar, under View, select Show Path Bar. On the Finder window, at the bottom, the "path" to the Documents folder appears. For you, it should say something like
Macintosh HD > Users > 54willow > Documents
That is the path to your Documents folder. Each level on the path is a folder. You can click on other "shortcuts" in the Finder window's sidebar, like Music. This shows your Music folder, which has its own path. Your files are organized into levels of folders (directories). Finder is the built-in application for navigating this structure, so you can "find" your stuff.
but very often, instead of the video flowing in smoothly and without stop, it will stop every few seconds and I get the spinning ball.
There are several factors. One is your connection speed to the Internet. Another is the connection speed of the video's host to the Internet. And the size of the video makes a big difference. If the video is lower resolution, it is smaller in size. If the video is "HD," it is much larger. Streaming (downloading and playing at the same time) larger videos smoothly takes more connection speed (at both ends). The "traffic" on the Internet is also a factor; the video may play better late at night. If your computer is really old and lacks performance to view the video smoothly, that can be a factor. But that will not be an issue with any recent computer; even cheap mobile phones can smoothly stream HD videos these days...
If you experience the "spinning ball" with a particular video, one thing you can do is wait until it downloads completely. Then the video's data is on your Mac, and it will play smoothly. Your Mac is just playing the video, not streaming it.