As you drive, tires are constantly being worn down. They should be regularly inspected and replaced when it is time to do so. Tires will wear down at different speeds for different people, based on vehicle type and how and where the vehicle is driven (speed, driving conditions, maintenance, driving habits, temperature, etc.).
A tire is considered “worn out”, or “illegal to use” when the tread wears down to a recommended minimum indicated by a “wear bar” (you can actually see a solid bar of rubber across the width of the tread). However in most cases to maintain best traction and stability in both dry and wet conditions tires should be replaced before reaching that point due to their significant performance fall-off past 50% of remaining tread depth. Consult your tire professional for more information.