Air is a fluid because the force needed to deform it depends on how fast it is deformed, not on how much it is deformed. This differs from a solid, where the force needed to deform it remains the same whether it is done quickly or slowly.
An object moving through air experiences forces similar to what it would experience moving through another fluid, such as water. For example, the faster the object moves, the more resistance it encounters. This is called fluid friction, or air friction, if the fluid is air. Airplanes provide a practical example of air's fluid nature. It is because air is a fluid that aircraft wings generate lift as they are forced through air at high speeds.