An epitaph, of course, is the set of words written on someone's gravestone. Shakespeare, being a great writer, (the greatest writer of all time?) wrote his own. It is quite strange. Let's look at the text first:
Good Friends, for Jesus' sake forbear,
To dig the bones enclosed here!
Blest be the man that spares these stones,
And curst be he that moves my bones.
Shakespeare apparently had a profound respect for the dead in that he was very upset by the current practice of "disposing" of dead bodies and skeletons and graves and coffins that have been at rest for a while in order to simply make "more room" for the new dead. One has to wonder if Shakespeare felt this way because of the vast number of characters who died due to severe tragedy during his plays.
In regards to the playwright's specific epitaph, of course Shakespeare felt as strongly about his own bones as he did about others; therefore, he felt called upon to make this statement. Paraphrased (Shakespeare begins with a kind greeting) it basically says, "Friends, for God's sake, don't even consider removing the bones that lie here. I offer a blessing to those who heed my warning and a curse to those that don't."
If you would like to know a bit more about the specifics of Shakespeare's epitaph, Shakespeare is buried in Holy Trinity Church in Stratford, England. The soul of Shakespeare (while it was still on earth) requests that his remains STAY at Holy Trinity Church in Stratford, England. Luckily for Shakespeare, even as late as 2008 when Holy Trinity church was renovated, those particular bones weren't touched.