This filter is used in telescopes for observing the moon. The moon is so bright under magnification by a telescope that details can be washed out, making it difficult to observe interesting details. In addition, too much light can mess up your night vision, something you want when looking at the night sky through a telescope. Think of this as "sunglasses for the moon". (However don't forget to remove it when looking at other objects or they'll be too dark!)
This is the one that you want for typical small powered telescopes - defined as 5" in aperture or less for the primary, or objective, lens. The more commonly available 13% transmission filters are for the "big boys" - telescopes that have an aperture of over 6". If you use a 13% filter on a small telescope you'll be filtering out too much light and the moon will appear way too dark. There are also variable transmission filters that use two polarizing lenses. But they are way more money and most backyard family telescopes are well served by this filter alone.
One note: this filter is used for eyepieces that are 1.25" in size. This is one of the most common sizes for telescopes but check yours to be sure first. It does not have to be an Orion telescope - any brand will do as long as the eyepieces are 1.25". Also make sure that your eyepieces have threads on the bottom inside of the tube, as this filter screws into the bottom of your eyepieces.
A finder scope is a low magnification, wide field of view telescope which is used to help point the main telescope at your target in the night sky. This one is an excellent value with bright images and views oriented the same way that most star charts are printed. Although I don't have a scope from the same manufacturer, it was very easy to fit it into the mount on my scope. I couldn't be happier!