It’s the null conditional operator. It basically means:
“Evaluate the first operand; if that’s null, stop, with a result of null. Otherwise, evaluate the second operand (as a member access of the first operand).”
In your example, the point is that if a is null, then a?.PropertyOfA will evaluate to null rather than throwing an exception – it will then compare that null reference with foo (using string’s == overload), find they’re not equal and execution will go into the body of the if statement.
In other words, it’s like this:
string bar = (a == null ? null : a.PropertyOfA);
if (bar != foo)
{
...
}
… except that a is only evaluated once.
Note that this can change the type of the expression, too. For example, consider FileInfo.Length. That’s a property of type long, but if you use it with the null conditional operator, you end up with an expression of type long?:
FileInfo fi = ...; // fi could be null long? length = fi?.Length; // If fi is null, length will be null