The stone is referred to as either the Trystan (Tristan) Stone or the Drustan Stone and is situated on the side of the road between Par and Fowey in Cornwall, where there is a small lay-by.
On the north side of the stone a raised 'T' shape can be seen. This is an old version of a Christian Cross known as a Tau. You might just be able to make out the 'T' in the photo above - obviously it has weathered somewhat over nearly 1500 years.
On the south side of the stone 6th century letters can be seen (above) - again well weathered but they translate as 'Trystan here lies of Cunomorus the son.'
Cunomorus was Marcus Cunomorus - King Mark of Cornwall in the love story of Tristan and Isolde.
I mentioned King Arthur. The connection is that Tristan (or Trystan) became one of the Knights of the Round Table.
As for the Tristan and Isolde story this is how one of the many versions goes:
Isolde married King Mark of Cornwall, but had an affair with Tristan. This continued even after her wedding. King Mark eventually found out about the naughty goings on but forgave his wife. Tristan, however, was exiled from Cornwall and he moved to the court of King Arthur.
Tristan later travelled to Brittany in France, where he met Iseult. He was said to be attracted to her because of the similarity of her name to that of Isolde. He did the honourable thing with Iseult and married her - but in his heart of hearts he continued to love Isolde - ahh!