An echidna is a small mammal with sharp spines all over its body, kind of like a porcupine. When a hungry predator comes near, an echidna will try to protect itself. One way it does this is by digging quickly into the ground with its strong claws.
Once the echidna has dug itself into the dirt, most of its body is hidden underground, and only its sharp spines stick out. These spines act like a shield, making it very hard and dangerous for the predator to bite or hurt the echidna. The predator usually cannot dig the echidna out easily and will often give up and look for easier prey.
So, when an echidna digs into the ground, it is using this trick to protect itself and stay safe from hungry animals that want to eat it.