This topic is more relevant to the CCIE SP track so I will just briefly go over the concept of this.

801.1Q-in-Q tunneling AKA Q-in-Q is a method of passing VLAN traffic across a WAN. At a simple level a SP switch tags incoming frames with an extra 802.1Q header and its passed through the WAN, where at the egress port of the SP network the tag is removed. The customer frames then have their original 802.1Q headers preserved after traveling across the WAN.

Q-in-Q allows customer networks to tagged frames across a shared SP WAN, other layer 2 protocols such as CDP and VTP are also allowed and it does not matter if the customer does not have unique VLAN numbers.