Silverfish are hostile mobs that spawn naturally in Strongholds. Their health is low so they can be killed easily.
Silverfish spawn naturally in Strongholds. They also spawn in the End Portal room in a Silverfish Spawner. In Strongholds, there is a block named "Silverfish Stone". If the player mines that block, a Silverfish will pop out and attack the player. If the Silverfish stays alone for sometime, it will return inside the stone.
Due to their low health, Silverfish can be easily killed with a Wooden Sword or above. However, like Zombie Pigmen, if the player provokes a Silverfish, then all the other Silverfish that are close will attack the player.