What does (!) dohere?

[Ziad22]

What does (!) do here?
(!a ) Means not 500?
or it change the value 500 ( true ) to false?

[DaveLembke]

Its a function that is inverse so instead of true false or instead of false true as seen here

https://teamtreehouse.com/community/what-does-the-means-if-its-before-a-variable