In an electronic system we can have different components performing different jobs, and combined they form a circuit.
In a typical circuit we have a voltage source, and we have devices that have some utility.
The voltage source can be a battery or for example an Arduino board providing power.
Let's say the voltage is 5V. We have a positive pole, and a negative pole.
If we connect those 2 poles directly using a wire, we're going to have a short circuit.
A short circuit is a very bad situation, and one of the most common causes of issues in electrical circuits, and in practice we take all sorts of precautions against this, because it can cause heavy damage to our devices and components, making them unusable.
Even in the case of a 1.5V battery, the amount of current flowing between the positive and negative poles can cause a fire (I do not recommend trying but this is a cool trick to light a fire when you forget the lighter in your camping trip).
Imagine what could happen with a larger battery. Some batteries (including the ones used in your cell phones) will explode if short-circuited.
Long story short, avoid short circuits in any possible way and pay attention.