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It is a Battle Honour.
The award of a battle honour to a ship is "..intended to foster esprit de corps among their officers and ships companies, who are thereby encouraged to take a personal interest in the exploits of....ships of the same name which have distinguished themselves in the past".
Battle honours record the actions or campaigns that ships took part in, and have the same status as battle honours awarded to British regiments. In the British army each regiment will have a standard, with the battle honours shown (the Royal Marines famously have just one - 'Gibraltar' - as they claim to have fought in so many actions, they only note the first, gained in 1705).
In the Royal Navy the battle honours accumulated by ships are displayed on a teak board, with scrolls carrying the name of the honours gained by ships of the name. The size of the board will obviously depend on the number gained, but is typically 6ft x 5ft. The ships badge will be an integral part of the board, with the names of the honours picked out in gold.
Ships accumulate honours, so they will display all past honours awarded. If my memory serves me, I think 'Warspite' had the most - with 25 battle honours, starting with 'Cadiz 1596' through to 'Biscay 1944'.
In the case of Mahratta, she had just the one, Arctic 1943-44.
This was awarded for ships that saw service between 1941 and 1945 within the Arctic Circle, except for the coastal waters of Norway and all those ships which were employed as escorts or in support of, convoys running to and from North Russia.
Any future ship that may carry the name would be allowed to display a board with the honour.