“Reinforcement” tab allows any clan member ranked “Veteran” or higher to send reinforcement request to all of her clan allies. You can write a message to your allies in the request interface, as well as indicate the time during which the request will be active. Your allies will see the request in their “Reinforcement” tabs and will be able to aid you.

Note that in order to send or receive reinforcements, you need to build a Consulate. You can read more about this and other buildings here.