This is not just any stone. This is....

...a coffin stone!
The notice has got rather weather-beaten, so for anyone who doesn't know about coffin stones here is a short explanation.
Cumberland and Westmorland used to have fairly small populations and fairly poor roads (often just rough tracks). At first many of the scattered communities did not have a church so when there was a death the coffin had to be transported to the nearest church for burial. This often involved being carried and along these routes there were suitable stones to allow those doing the carrying to have a rest.
This stone is on the 'coffin route' between Ambleside and Grasmere. Grasmere has a very old church but Ambleside had to take their dead over this route for many years.
There are serveral other coffin routes in Cumbria and in other counties too.