Canon 300mm f4.0L IS USM is an Auto Focus (AF) Telephoto Prime lens with 35mm FF max sensor coverage. When mounted on a 35mm FF sensor camera, it will give you an FF equivalent focal range of 300mm. With a max aperture of f4 at 300mm, this is a moderately fast lens.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Canon 300mm f4.0L IS USM is not a weather sealed lens, so you have to be extra careful against rain and dust.