Don’t. Even if you understand what you’re doing, someone else won’t and your family will die in a house fire.
Male-to-male end cables allow for the two leads of an open male end to make contact with anything conductive (including your flesh, causing some pretty gnarly burns) to complete the circuit and start a fire. No conscionable electrician will make these for you. It is the dumbest solution to any problem you’re looking to solve. Don’t.
Just a reminder coming up on Christmas.
Nominally you can use it to plug a generators output into a household circuit, which will provide power to that circuit in cases of a blackout, saving you from needing to unplug everything critical and daisy chain 10 multiplugs to the generator.
It could also be used to connect two seperate household circuits together, if only of them is actually live for whatever reason.
In reality you shouldn’t his at all, ever. Just daisy chain the extension cords. If you forgot to isolate the circuit by flipping the main breaker (easy to do if there’s no power anyway, because of a blackout), and then the grid comes back on, your generator is gonna have a real bad time. And then there’s obviosuly the electric shock risk of using something like this.