This is a common expression among Navy Seals. And in my opinion one of the greatest personal axioms.
It simply means have a backup. If you have two of something, you will break or lose one and end up with one, if you have one, you will break or lose it and you will have nothing.
Its all about eliminating ‘Single points of failure’. This can be applied everywhere, from the personal equipment that you carry all the way to your retirement plan.