It is analogous to launching a ship, a specific process that has largely been discontinued in modern shipbuilding.
After the empty hull has been substantially completed, sluice gates are opened and the drydock fills with water.
The dock gates are then opened and the ship is pulled out by tugboat to a berth where the remaining construction continues namely fitting out.
The completed ship will usually return to drydock for installation of other equipment, propulsion parts, and the painting of its hull.
SS Great Britain designed by Isambard Kingdom Brunel was constructed in drydock and floated on 19 July 1843.