So, it's a matter of triggering/making events in Wc3.
JASS or JASS2 is the coding language that maps use and is interpreted by the game. I've heard that it stands for "Just another scripting syntax"/"Just another scripting system", which might not be improbable if you consider that Blizzard named other things here on default as "Just another Warcraft III Map" or "Just another Warcraft III Campaign", "Just another Warcraft III AI" etc.
The editor is a tool that should make it easier to create a functionable map archive file for the game. The graphical user interface (GUI) is a likewise simplification for Jass. Unfortunately, it may be better for beginners who have not been in scripting languages yet than to stand in the long run. The GUI is relatively slow and limits the possibilities of its parent. On the other hand, amateur mapmakers were more confident about it and spread, granting the community a lot of maps. GUI triggering is safer because of said limits and therefore has a more solid basis (and less disappointments in that way).
Well, Jass among programming/scripting languages is comparatively easy. It has only a few structures/keywords and is verbose. An obstacle is that the API (application programming interface), which contains the native functions, variables, types etc., is not provided within the normal editor, nor is syntax highlighting, auto-completing and the likes. Some devotees then developed extensions of Jass like vJass or Zinc and delivered help programs and precompilers that should make it easier and that would transform code of the new style into Jass. Before I used JNGP (Jass NewGen Pack), which hacks into the normal editor to expand its functionalities and to still use its simplicities and which unites different helping tools in this interface, and before I knew where I could find the API in Wc3's files, I had informed myself at this online resource
here. There are also some tutorials.
Aside from that, converting GUI triggers to Jass can help to get clues. I think these ones that start with Jass do not really have a problem with it, at least not more than they would have with learning GUI from scratch.