What is reprojection, and why do we use it in remote sensing?

Answer 1

A projection is a system which is used by GIS to bring a map from a 3d scale to a 2d scale. Think of it like taking your globe and cutting it up so you can place it on a flat table. Because the globe is round you're going to have to make minor adjustments so that it fits. There are multiple different types of projection and its ultimately up to user preference.

Reprojection is simply converting one file's project to match another files. You use it in remote sensing so that you know exactly how the data is being presented to you.