The presence of multiple stereocenters in carbohydrates makes it a challenge to accurately describe the structure of such a molecule in any way that is reasonably simple. Two conventions have been developed to try to simplify the situation. The Fischer Projection is a convention to depict the correct configuration of a carbon as simply as possible. This is straightforward, just definition.
Most students have the most difficulty when dealing with multiple stereocenters.
Remember that each stereocenter in a Fischer projection is viewed separately. We can also recognize that there are several ways to move substituents around in a Fischer projection and keep the same configuration. Rotating by 180 gives back the same structure. Exchanging any two positions gives the opposite configuration, but pairwise exchage of two sets gives back the same structure. The Haworth projection is a way around this limitation that does not require you to try to convey the complete 3D image of the molecule.