Imagine that you have four buckets with various stuff and the person in the next room needs you to bring one of the buckets but has to tell you which one by way of two lights that they can either turn on or off.
A B C D
You have two assistants, each of which as one of the buckets in each hand.
The first light tells the assistants which of their two buckets to hand you (the first assistant puts a bucket into your left hand and the other assistant puts one into your right).
The second light tells you which of the two buckets you now have needs to be passed through the door.