A function starts with it's name followed by arguments (also called parameters) separated by commas inside parenthesis.
function_name(argument, argument, argument);
Functions are used when scripting games, they are like drag and drop actions except in code so that you can use a lot more of them and use extended capabilities of ENIGMA. General procedures for documenting functions can be found on MediaWiki help.
Here you can obtain information about functions in ENIGMA, they are grouped for your convenience. To find if a function has not yet been implemented please see Unimplemented. There is also an article on deprecated GML behaviour.
These are links to the function category pages so you can find them all on one page and search for them.