Difference between Obj.Prop.AnotherProp and Obj.Prop?.AnotherProp

Question

var Obj = { Prop : { AnotherProp : "Hello World" } },
    log = console.log;

log( Obj.Prop.AnotherProp ) 
log( Obj.Prop?.AnotherProp ) // What does Prop`?` do?

// From Google Chrome and Node output is 
// Hello World
// Hello World

I am confused about calling Object Properties.