The text plays on the dual meaning of 'Cheerio, see you soon.' While 'Cheerio' is a casual British farewell and 'see you soon' is a common parting phrase implying a future meeting, the humor arises from the fact that this is written on a gravestone, where the expectation of a future meeting is ironic and macabre.
Epitaph on a gravestone : Cheerio , see you soon .