When does one Compile a map or build a mod? What do they do? I've always been confused on these apparently important functions on the Launcher.   ???

Depending on what mod tools u are using the process is slightly different. Tell us which mod you are using. I would always refer to wiki or youtube if I am stuck , and these forums . following a tutorial to get the basics down. Once u know it is a simple process.

Once you have worked on your map file save it . Build Iwd and ff (slightly  different process for ugx mod ) then compile the map  . If you are a beginner I recommend ugx mod for included weapons and game modes.. This is your choice of course. Using ugx mod will give u a good understanding of the mod folder and what is required. This way you can focus on learning how it works .