Vim and various plugins generally use known file extensions to determine the filetype of a file. This is important because it is how Vim decides which filetype-specific settings to enable, such as syntax highlighting.
If I am editing a file such as build.boot, Vim is not going to know that
its filetype should be set to clojure. The build.boot file is full of
clojure code though, so I'm losing out on syntax highlighting and so forth.
I can settle for manually setting the filetype to clojure (e.g. :set ft=clojure) each time I open up the file.
Or I can use a modeline setting. By including a comment at the top or bottom of the file specifying the filetype setting, I can ensure that each time I go to edit the file, the appropriate filetype will be set.
That modeline comment will look something like:
; vim: set ft=clojure:See :h modeline for more details.
h/t Brian Dunn