Raycast issue

Okay, so, I found this, https://bukkit.org/threads/using-rays-to-quickly-and-accurately-detect-hitbox-collisions.441877/ , which I'm trying to use for a plugin, but it says "CraftEntity" cannot be resolved to a type. How do I fix this, and why did it work for other people but not me? Also, what does "CraftEntity" even mean?

 

What's an NMS class, and how would I add the server? Do I just add the entire folder?

 

...Looking at it now, I may have posted this in the wrong place.
I'm really new to the whole plugin stuff (and servers, too), and never understood the connections/differences between spigot/bukkit/paper and the like. The problem is that I don't have a server jar because I'm running spigot. The code seems to have a lot of Bukkit stuff in it (imports from org.bukkit.net and stuff), so I thought this would be the right place.
Sorry for the inconvenience, then, I'll look for help other places.

Edit: Wow yeah spigot has it's own forums. I'm really sorry.

 

Oh, what? I do have the spigot.jar in the build path, though.

Edit: No, nevermind, I didn't. Thank you for your help!

Edit 2: I added it but it didn't work, it still says that CraftEntity cannot be resolved to a type.

Edit 3: After restarting my IDE, it works. So, thank you, still.