You're looking at a ternary operator. The ternary operator ?: works like a simplified If/Then/else statement.
The first part precedes the question mark, and is an expression that evaluates to true or false. In your carse, it's "url.Contains('.')". The part that immediately follows the question mark is the result if true, and the part follows the colon is the result if false.
So, in your case, "url.Substring(url.LastIndexOf('.'))" is returned is true, and "" is returned if false.
For more information on the ternary operator,
have a look at this.