Romans used the testudo formation to protect themselves from the enemy’s fire. The legionaries resembled a turtle-covered shell – hence the name. It was a compact rectangular formation in which the legionnaires (usually 27) from the first row and from the sides of the formation held the shields in front of them or from the side of their exposed side, while the legionaries from the inner ranks held the shields horizontally above each other and over the legionaries of the first and side ranks, thus creating a shield of the whole formation covering themselves from enemy shots.
The legionaries, who were moving in such a column during the battle, resembled a turtle-covered shell – hence the name.
Apparently, the Romans checked the testudo stamina by throwing chariots at it. You can read about such a formation check in the book of Peter Connolly “The Roman Army”.
In turn, Cassius Dio mentions in the “Roman History” that the testudo formation was strong enough, compact and durable, that it was apparently able to withstand the weight of a horse running on the shields, and even a towed car.