There are several kinds of mountain bike clothing available. Not every item will be absolutely necessary for you, but if you do have the proper apparel, you will be more comfortable and aerodynamic as you ride. The overall clothing is designed to keep you comfortable while exerting yourself. It is also durable, which is important. Regular clothes do not work as well because the fabrics are less aerodynamic and more destructible.
Beyond the basic mountain bike shorts and shirt, there are several other important pieces of apparel. One piece of clothing that you do not want to go without in any circumstance is the helmet. This will protect you against falls over rough, rocky terrain. Some of these helmets are also full-faced.
Mountain bike gloves are distinct in that they are made of a heavier construction, sometimes with padding for the knuckles. If you know you are going to be riding in some particularly adverse or muddy conditions, you might want to invest in some glasses or goggles to protect your eyes against flying debris. Finally, mountain biking shoes typically have gripping soles, which are meant to be effect on the ground when you have to walk your bike past obstacles in places you cannot ride. You might or might not need all of these things, but they all provide potential benefits.