A raw image file is essentially the data as it comes directly from the camera's sensor with absolutely minimal processing. To get from this raw data to, say, a JPG, the camera manufacturer makes a huge number of assumptions about your preferences, and these are "cooked" into every JPG that it produces.
Once this is done:
(a) trying to change the assumptions built into the JPG almost always introduces artifacts and errors. For example, trying to make a white balance correction starting from a JPG will never be as good as if you started from the corresponding raw data file.
(b) you lose a lot of the dynamic range and image quality that actually is present in your image. For example, a JPG only has 8 bits of information for each color, whereas a raw data file typical has between 12 and 16 bits of info for each color.
Almost no one who is serious about photography and does any amount of post processing of their images uses only JPG. They use raw files and then do the conversion of the raw data in their computer (eg, in the ACR (Adobe Camera Raw) part of Photoshop and Lightroom.
Your camera has the option to save your pictures as (1) JPGs only, (2) RAF files (Fuji's raw data format) only, or, (3) a JPG and a RAF file for every image. Even if you aren't doing your own post processing right now, you might want to do this in the future, o,r if you hire someone else to do this work for you, they will always get better results if you set your camera to always save images in both formats. You have an extremely capable camera -- you should use features like this. A nice summary of its capabilities can be found here:
http://www.dpreview.com/reviews/fujifilm-x-t1
A very nice introduction to raw image files is available on Wikipedia. There is a lot of info in that article, but you only have to read as much of it as you want:
https://en.m.wikipedia.org/wiki/Raw_image_format
HTH,
Tom M
