A plain bearing is simply where two surfaces pass each other, either in rotation or sliding. Sometimes a special alloy such as "Muntz metal" is used for the bearing surfaces. Lubrication between the two surfaces is essential, to prevent them pressing against each other. The pressure within a correctly lubricated bearing produces a wedge of oil, that the rotating or sliding parts actually ride on!
It is called a "Bearing" because it has to bear the weight of the moving components, whilst still allowing free movement and not eroding too quickly!