A strand of christmas lights resembles an extension cord, but they tend to be made of smaller gauge wire and obviously have little sockets for tiny light bulbs spaced along them. They typically have 2-prong male plug on one end, often with a 2-prong female pass through on the back so you could plug more than one strand into the same receptacle, and they usually end in a female plug so they can be daisy chained.

Sometimes, when installing them on a house or something, the person installing them may not pay attention to which direction is which, and end up installing them so that the female-only end is near where they intended to plug them in. So instead of pulling them down, or running a long extension cord, they go to the hardware store looking for a male-to-male plug adapter.

Power plugs and sockets are gendered for a very good reason; the female receptacle keeps the energized contacts protected inside, and the male plug's contacts should only be energized when plugged in and their outer shells protect them. A male-to-male cable when one end is plugged in and the other is free now has exposed mains current just waving around in the open air ready to kill someone. And, on a smaller note with christmas lights, they usually have a fuse built into the plug, and plugging them in backwards bypasses this for at least the first strand, so it's technically 102.7% unsafe to do this.

The other thing a male-to-male adapter or cable is sometimes used for is to attach a portable generator to your home's electrical system by just plugging it into an outlet, especially during a power failure. They do make what are essentially special male receptacles I think mainly for the RV industry for attaching generators like that, most houses won't have these. Plugging it into a normal wall socket will actually work, but 1. you have bypassed the breaker panel, so the breakers no longer provide over-current protection. You could overheat the wires in the walls and burn down the house. 2. there's a possibility that you're feeding electricity to the entire house through the breaker box and even out to the transformer, which means the lines could be energized for linemen working on them. Throwing the main breaker might prevent that? They make switching gear designed for buildings with their own backup generators that can either manually or automatically sever their connection to the grid when on internal power, but again a doofus trying to make one of these cables probably doesn't have one of those.