Some photographers shoot RAW. Some shoot JPG. Some hop back and forth depending on the day/job/requirements.
If this is gibberish to you, I'm going to show you the difference right now. But first, let me explain the situation.
My husband and I were on a remote hiking trail in Hawaii. 4,000+ miles by plane, an hour of driving, and then 6 hours of strenuous backpacking to get to this spot. The trail was narrow, muddy and shaded by trees. At the head of the valley was the Hanakoa Falls that I had been dreaming about for days... weeks maybe. It was my goal to swim at that waterfall, come hell or high water.
We made it there, and into the icy water I went. My husband was on shore to take the photo, but as he grabbed the camera it started to rain, which quickly turned into a downpour. He pressed the shutter without a chance to glance at the settings, which I had set before the clouds came over us. The resulting image on the left is what we got... and as you see, it is incredibly underexposed.
We scrambled to get the camera and our towels under cover, but the rain didn't pass. It continued on for the rest of the night.
This moment -- me swimming at the base of a 440 foot waterfall... by myself... in the rain... 4,000+ miles from home-- almost didn't turn out. This is one of the only photos we have from what is honestly the highlight of the entire trip! A once-in-a-lifetime kind of moment, if you will.
But when I got home, I used Lightroom plus that RAW file, and was able to bump it up 3.15 stops to get the image on the right.