This variable contains link types and how they behave on follow (i.e when someone opens the link with C-c C-o), export etc. A type of a link is determined by the string before first ":" in it. e.g "https://bitspook.in" has https type, "file:///etc" has file type.
This function is used to add new link-types (and also to add new behavior to existing ones). You can check the docs for org-link-parameters (with C-h v org-link-parameters) to see arguments provided to each type of callback.
Setting display-buffer-alist(L2) ensures that the process output buffer that get created when mpv is launched don't come in focus. I have chosen to let the shell-output-buffer be created to allow having a peek at the command output, and to kill the process if needed.
Recently I've been building a tool (cl-ownpress) which will make blogging a zero-effort activity for me. First step in this is to publish a subset of my notes. I have already built a habit of judiciously taking notes. Simply publishing a subset of my notes will enable maintaining an active blog without doing any "work".