In real life
In real life, tombstnes are as their name implies stones that mark the head of a grave. A normal tombstone would say "R.I.P.", the person's full name, their date of birth and their date of death. Sometimes, a further epitaph might be added.
Haunted Mansion tombstones
In the Haunted Mansion, however, the tombstones usually have funny epitaphs that explain the cause of death of their occupant in rhyme. This is likely due to the ghosts, due to having fun in the afterlife, having stopped to take death seriously.