From https://autohotkey.com/docs/misc/WinTitle.htm#ahk_class
A window class is a set of attributes that the system uses as a template to create a window. In other words, the class name of the window identifies what type of window it is.
In other words you can use it to identify windows of the same type, if you open notepad the title will be Untitled - Notepad if you save it to temp.txt the title will become temp - Notepad. ahk_class on the other hand will always remain Notepad.
The second example will work if you replace it with #IfWinActive ahk_class MSPaintApp because that’s the class of mspaint.
Usually you find ahk_class using Window Spy and then use it in your scripts. If you don’t have Window Spy you can use the following hotkey:
#1::WinGetClass, Clipboard, A ; Will copy the ahk_class of the Active Window to clipboard
After you found it you can use it in any place you can use window title for example instead of writing WinActivate, Untitled - Notepad you can write WinActivate, ahk_class Notepad.