BoxOfSnoo: Git is sooo much nicer. You maybe know this already though.
drmike: Find me the download link on this page:
https://github.com/Pathoschild/StardewMods/tree/develop/TractorMod As was mentioned, that has nothing to do with git. However, to answer your question:
Go back a level to
https://github.com/Pathoschild/StardewMods and click "Clone or Download". Copy the URL and paste it after "git clone " on the command line (you will need git installed of course).
OR, go to "Releases" on that page (just above the green line) and download whatever the dev prepared as a release.
GitHub takes a bit of getting used to, of course, but it's usually all there. Gists are super handy too.
I've been much more of a Bitbucket user, but Github is fine.
git itself, on the other hand, is amazing for managing code as a dev. For me there's no good reason not to create a git repo and commit like a maniac even if I'm only working locally. I even use git on my iPhone and iPad for storing notes on bitbucket.
If what you're trying to do is check out ONLY that one folder (the value of which is usually questionable, given how interrelated most codebases are...), you want to do what's called a "sparse checkout". Possible, but probably not worth the effort.