A sauce that coats the back of a spoon

In a recipe I read recently, I came across the expression “reduce the sauce until it coats the back of a spoon.” Can you explain exactly what that means?

This technique is most often used to test whether a custard sauce is done. You stir the sauce with a spoon and then immediately trace a line across the back of the spoon with your fingertip. If the line remains visible, the sauce is thick enough; if it doesn’t, the sauce needs to be cooked a little longer.