system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does it mean for a battery to 'short-circuit'?
gpt
In the "-"-terminal of the battery, there are lots of tiny particles called electrons, that desperately want to get to the "+"-terminal of the battery. When you use the battery to power a lamp, for example, you allow them to go there, but you put something in their way that slows them down and takes their energy to do something else, in this case produce light. Now, when you connect both terminals, you're allowing the electrons to get to where they want without anything in the way to slow them down, so all try to get to the other end at once, and there is no controlled way for them to loose their energy. All that energy still has to go somewhere though, so it dissipates uncontrolled, most of it in the form of heat. This heat can damage the damage the battery, and might even cause it to explode.

This is what you call a short-circuit, connecting the two terminals of a power source directly to each other with no consumer between them to control the energy flow.