Berth is the required minimum level of a Spaceport required on a planet before a ship of a required Berth size can be sent from the planet.
At the current time most ships in the game have a Berth size of 1. Higher Berth sizes have only been placed on various types of Cargo Ships, and on Scows. Future revisions to ships may add higher Berth requirements to some of the Combat Vessels.
Playing Tips: Try to keep at least one Spaceport on each of your planets at the maximum level that you can support in order to be able to use the ships with the larger Berth requirements.
Also ships built prior to the addition of the Berth requirement did not have this particular stat added to them and can still be used on any planet regardless of the spaceport level. However such ships will continue to grow more scarce as time passes but can currently be found for very cheap as many players are clearing their dock spaces to make way for building the larger capacity ships.