So imagine you are playing in a sandbox and you dig a big hole. Now imagine there are two groups of ants living underground - Group A and Group B. They both want to be the only ones living in the sandbox, so they start digging tunnels towards each other.
Soon they meet in the middle and start to fight - biting, scratching and pushing each other. They are having a battle right beneath your feet!
Just like ants, armies in the past also used to dig tunnels to attack each other from underground. This was called "battle beneath the earth" or "tunnel warfare". They would dig tunnels under enemy lines, sneak in, and attack them from below.
It was a very dangerous way to fight as the tunnels could collapse and the soldiers could get trapped underground, or run out of air to breathe. So it was a tactic only used by commanders who were ready to take big risks.
Luckily, nowadays, we have better and safer ways to fight wars, so we don't have to worry about battles beneath the earth as much.