next to the numbers in a script you can click and a red ball will appear ... what's the use of it?
Its purpose is to make a mental note of what line in a script you want to pin-point
A BREAKPOINT!
for debugging purposes, you can place a breakpoint so that one script execution reaches that line, it pauses and you can check to see what things happened on this line and the previous lines, even being given variables and their values and scopes.