Well a ring main is a pipe or cable connected in the form of a loop or ring.
The electricity or piped substance can then be fed in two directions to any point.
The purpose of this is illustrated in my sketch.
Suppose you have loads or outlets A, B and C fed from a source as shown.
If B is a heavy load than there will will a large pressure drop from the source to B.
So if outlet C or load C is fed only via B (the thick line) then it will be fed at low pressure or voltage.
If we now complete the ring via the dashed line the proper pressure or voltage is maintained at C.