What does mean "?" after variable in C#?

Check the C# operator list:

x?.y – null conditional member access. Returns null if the left hand operand is null.

x ?? y – returns x if it is non-null; otherwise, returns y.

So helper?.Settings will return null if helper is null otherwise it will return helper.Settings

if helper.Settings is not null and helper.Settings.HasConfig is not null then it will return the value of helper.Settings.HasConfig otherwise will return false.

N.B: if helper?.Settings is null then NULL reference exception will occur.

Well, ?. is a null-conditional operator

https://msdn.microsoft.com/en-us/library/dn986595.aspx

x?.y

means return null if x is null and x.y otherwise

?? is a null-coalescing operator

https://msdn.microsoft.com/en-us/library/ms173224.aspx

x ?? y

means if x == null return y, otherwise x

Combining all the above

helper?.Settings.HasConfig ?? false

means: return false if

helper == null or
helper.Settings.HasConfig == null

otherwise return

helper.Settings.HasConfig

The code without ?? and ?. if can be rewritten into cumbersome

if (!(helper == null 
        ? false
        : (helper.Settings.HasConfig == null 
             ? false
             : helper.Settings.HasConfig)))