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This might be answered better in https://learningnetwork.cisco.com/welcome, however Q in Q is basically a frame with two VLAN tag headers. It's often used with L2 MetroE providers. It allows the provider to have their own VLANs distinct from customer VLANs running across their network. For example, on the MetroE provider network VLAN 5 contains all customer X's traffic while VLAN 6 contains all customer Y's traffic, yet both customer X and customer Y are using many VLANs, possibly including VLANs 5 and/or 6.
