The ! part is meant for API scripts (Pro subscriber feature) - these buttons are meant to be used for the API, but users found a way to hack them for normal macro and abilities. Explaining what it does is tricky, because it builds on several different bits of arcane roll20 lore. Firstly, commands of this nature [something](something else) are intended to be used by the api. The square bracket contains a visible label, the round brackets contain an API command. API commands must begin with !, and be a single line. Roll20 treats any lines that start with ! differently - they get sent to chat, where they are intercepted by the API engine and so dont actually appear in the chat. So if you did this: !#Macro That macro trigger #Macro would appear to be ignored - it gets intercepted by the API before the dice engine can act on it. Thats where this comes in 
 In html, every character can be represented by a html entity , which is a weird string of characters like 
 This particular set of characters represents a line break. Now you cant do this [Critical](!
#critical-hit) because that linebreak breaks the button. But the html entity doesnt actually get treated as a line break until the roll20 parsing engine acts on it, and it does that before the API engine intercepts the call. So you have this: [Critical]( !
#critical-hit ) .At this point, the bit in ( ) is just text. Roll20 doesnt do anything with it. Then a player clicks the button and several things happen: First, the roll20 engine reads the text, sees the html entity and processes it into a line break. Now it has the text !
#critical-hit So thats a two line macro, which the roll20 engine can handle, and acts on each line in turn. The first line starts with !, so gets sent to the API engine. But its empty so it doesnt do anything. The second line is a standard macro call, which gets run now. The html entity exploits the way roll20 parses text sent to chat: the contents of a command button are left completely alone until someone clicks on the button, and then the magic happens. I hope that helps understand. It is pretty weird :)