So, earlier you learned about the polarizer and how essential that filter is. This filter, the neutral density gradient filter, isn't necessarily essential but it is the secret weapon of professional landscape photographers. It lets them balance the exposure between the ground and the sky to capture a range of exposure which, without it, their camera could never pull off (it's either going to expose for the ground or for the sky, but not both at the same time). For example, let's say you're shooting a landscape at sunset. If you expose for the sky, the sky will look great but the ground will be way too dark. If you expose for the ground, then the sky will be way too light. So, how do you get both the sky and the ground to look right? With a neutral density gradient filter (a filter that's dark at the top and smoothly graduates down to transparent at the bottom). What this essentially does is darken the sky (which would have been overexposed), while leaving the ground untouched, but the brilliance of it is the gradientit moves from darkening (at the top of the filter) and then graduates smoothly down to transparent (on the ground). That way it only darkens the sky, but it does so in a way that makes the top of the sky darker, and then your sky gradually becomes lighter until the filter has no effect at all by the time it reaches the ground. The result is a photo where both the sky and ground look properly exposed.
