When it comes to cruising, the term "berth" can have several meanings. During the booking phase of a cruise, you may encounter the term as it refers to beds. In general nautical terms, the number of berths on a vessel is equal to the number of people who can sleep onboard. Most cruise lines now simply refer to all beds as beds, but upper bunks that pull down from the wall are sometimes still called berths or Pullman beds.
In a more common usage, a ship's berth is its parking spot. Whether that spot is at a dock or an anchorage in or near the port, each ship is assigned a place to park in the harbor. Cruise lines pay for the privilege of parking in a port, with the price of the berth often tied to the size of the ship. Mega-ships can require as much dock space as two small cruise ship berths.
In nautical navigation, "berth" is used to describe the space surrounding a vessel. Thus the expression "give a wide berth" translates to giving plenty of space.