(This is a duplicate, but it's hard to search for, so I'm happy enough to provide more another target for future searches...)
It's the null-coalescing operator. Essentially it evaluates the first operand, and if the result is null (either a null reference or the null value for a nullable value type) then it evaluates the second operand. The result is whichever operand was evaluated last, effectively.
Note that due to its associativity, you can write:
int? x = E1 ?? E2 ?? E3 ?? E4;
if E1, E2, E3 and E4 are all expressions of type int? - it will start with E1 and progress until it finds a non-null value.
The first operand has to be a nullable type, but e second operand can be non-nullable, in which case the overall expression type is non-nullable. For example, suppose E4 is an expression of type int (but all the rest are still int? then you can make x non-nullable:
int x = E1 ?? E2 ?? E3 ?? E4;