swear on mother's grave: o que isso significa?

While watching The Office, a very comic american sitcom, the manager of the company was trying to calm his employees by saying there wouldn't be any downsizing. So as a way to guarantee he is being serious, he swore on his mother's grave he was telling the truth.

When you swear something on you mother's grave, you're stating something very earnestly, in a way that you're putting your mom's death at stake to guarantee you're being honest. Children do that all the time when they want to swear something and make sure other children will believe them. Adults can do it too, but mostly in a humorous way, like in the situation of The Office. That's all. Bye bye!