The Services menu (or simply Services) is a user interface element in macOS. The services are programs that accept input from the user selection, process it, and optionally put the result back in the clipboard. The concept originated in the NeXTSTEP operating system, from which it was carried over into macOS and GNUstep.
For example, it's possible to desktop search for a piece of text by selecting it with the mouse and using the service from Spotlight. Other central services are Grab for taking screenshots, and the system spell checker. The concept is similar to a GUI equivalent of a Unix pipe, allowing arbitrary data to be processed and passed between programs.
In my use case, I often need to clip a section of a text file, but not the whole file.
Before I moved over from the Green Elephant, I could do this easily by selecting the wanted text (from wherever it existed - separate text file, web browser, email app), right-mouse-button to reveal the contextual menu and then select "Add to Evernote" from the Services submenu.
I found this to be a really fast and efficient workflow and would love it if Joplin could implement this feature.