What is the meaning of [Speak well of the dead]

Speak well of them, for they cannot speak for themselves. ‘Death softens all
resentments’, wrote John Greenleaf Whittier, the American poet, ‘and the
consciousness of the common inheritance of frailty and weakness modifies the
severity of judgement.’ The Latin version is _De mortuis nil nisi bonum_ – ‘Of
the dead nothing but good.’