The Source is an alternate side to the editor, instead of the design in the source lets the user code the game, although it is recommended to use the design version for the editor, and the source is mainly used by expert developers. The source is easy to learn if you know basic HTML5. The difference between the Design and Source is that Design you don't need to know how to code and let's you drag and drop Code Blocks into the game and these blocks have code already in them that also lets the user edit them in their properties, but in the Source you have to code all of it in including properties.
While coding in the Source editor you need to know how to use Brackets/Tags. First you will encounter two things
<GameMaker3D> (You never erase these)
This is were you type your code.
</GameMaker3D> (You never erase these)
If you want to code somethings in you need to first put < then when you are done coding in you put />, < means beginning and /> means end, here's an example, if you want to add a cube you put <Cube collision=“1” static=“1”/>.