The keel of a ship or boat is the center structural piece or spine of the bottom of the boat that has the hull attached to it. The "laying" of the keel is the official first step in the construction of the ship and is celebrated with a ceremony attended by officials from both the cruise line and the shipbuilding company.
In the era of wooden ship building, the celebration surrounded the actual placement of the wooden backbone of the ship's bottom structure in the construction dock. Today's modern cruise ships are a composite of modules built in construction yards, not in a dock. The keel laying ceremony is held when the first module or set of modules are placed in the dock in which the ship will take shape as the modules are joined together and from which the completed ship will eventually be floated out.
For good luck, a pair of coins are sometimes placed under the keel or welded in place on the hull in a coin ceremony that may take place in conjunction with the keel laying ceremony.