RO/RO is an acronym (pronounced "row-row") for Roll-On / Roll-Off. It is sometimes written 'RORO' or 'RO-RO.' It is a term used to describe a particular characteristic or capability of ship, and sometimes of ports. Literally, it means the ability to load and unload cargo by driving vehicles directly onto and off of the ship. This is in contrast to most modern cargo ships, which are container ships and whose cargo is on- and off-loaded by dedicated cranes on the quay.
Typically, there are three major types of ships which are RO/RO capable. The first is automobile ferries, whose entire job is to quickly load, move and unload rolling vehicles. The second is automobile carriers - large cargo ships optimized for transporting cars (without their passengers) across oceans. The third is military sealift and PREPO ships. These latter are unique in that their cargoes are usually varied, and include vehicles of all sorts for both combat and logistics use. Although most such ships do require a port infrastructure to operate, at the extreme end are military landing ships which are designed to transport combat vehicles onto an unprepared beach.
Before RO/RO, all ships were loaded using cranes. RO/RO only really became important in the twentieth century as motorized transport came into its own, and not only the cargo but the vehicles themselves became items that were moved between shores.
DonJaime points out that after a particularly nasty disaster involving a capsizing car ferry, the term 'RORORO' came into sarcastic use - Roll On, Roll Off, Roll Over. Professor Pi surmises that this refers to the Herald of Free Enterprise capsizing and sinking.
This writeup is dedicated to leuryaks.