The C/AL Editor is the editor where you change the C/AL of the objects in Navision.
The C/AL Editor is available from all objects, but it requires that you have a full application developer license. The Codeunit object is the only object that only have the C/AL Editor.
The most important shortcut in the C/AL Editor is F5. This will show you the C/AL Symbol Menu, where you can see all "symbols" available, including both the tables/fields as defined by the object, the variables you have created specifically for the object and also all the standard C/AL functions, such as ex. COPYSTR.
From the Symbol Menu you can both see the syntax for the function, but you can also always hit F1 to show the content specific help on how you use the function.