What is the meaning of [Every family has a skeleton in the cupboard]

Every family has a guilty secret that it does its utmost to conceal from the
world.

> The widow and her three grown-up children came to England from Australia
last year. All four are so careful about what they say that nobody imagines
for a moment that they have a skeleton in the cupboard, yet the fact is that
the ­ ‘widow’s’ husband, a criminal of the worst type, is serving life
imprisonment in Brisbane.