Samyang 300mm f6.3 is a Manual Focus (MF) Telephoto Mirror Prime lens with APS-C / DX max sensor coverage. When mounted on a APS-C / DX sensor camera, it will give you an FF equivalent focal range of 450mm. With the max aperture of only f6.3 at 300mm, it is considered a slow lens.
Samyang 300mm f6.3 is not a weather sealed lens, so you have to be extra careful against rain and dust.