Docking is when someone cuts off part of an animal's tail or ears.
Sometimes, people want their animals to have a certain look or style. For example, some people want their dogs to look like they have a "bobbed" tail, which means a tail that's shorter than normal. So, they might take their dog to a person who can cut off part of the tail to make it shorter.
The same thing can happen with ears - people might want their animals to have ears that "flop" down instead of standing up, so they'll have part of the ear cut off.
Some people do docking because they believe it can prevent health problems or injuries. For example, some kinds of dogs can accidentally hurt their tails by wagging them too hard or getting them caught in things, so people might cut off part of the tail to prevent this from happening.
Docking can be done when the animal is very young, so they usually don't feel much pain. But it's also controversial because some people believe it's cruel to cut off part of an animal's body for cosmetic reasons.