The fact is: it’s very peculiar. The carving is an exact mirror image of a famous painting (by Poussin), in which shepherds appear to examine a tomb. Why the mirroring? No one knows, as the people of the time – it was made in 1748 – aren’t saying.
Then – this sounds crazy – a story grew up (probably fabricated of course) that the carving is a clue as to the site of a putative nearby resting-place of The Holy Grail (the ancient chalice that once contained the blood of Christ and which, the legend goes, ended up in Britain). The tomb in the carving suggests this idea, it is said.
Anyway, the last peculiarity is that there is an unexplained line of ten Roman letters carved into the plinth. You and I might think that it’s just a private inscription of abbreviations put there by the lord of the time; but lots of people have got very excited over it and insist it’s a secret code. In fact, a whole cyberspace buzz has grown up around it.
Anyway, if you want to know more, even Staffordshire County Council hasn’t been able to resist the ‘mystery’ and has a whole page devoted to it on the council's website.
See: Staffordshire & the Holy Grail / Shepherd’s Monument (BBC)