Literal Meaning

'Sickness worsens during the night' (one who says so) - covers the hot charcoal - burning in the fireplace - with ash

Implied Meaning

We must prepare for an inevitable occurrence when our resources are still adequate

Origin

Before the advent of matches, torches, or electricity, a family that had a sick person preserved the charcoal in the fireplace by covering it with ash, so that it could be used as light in case of an emergency