gluten is a protein found in wheat, barley, rye, and most oats. People with Celiac disease and gluten intolerance (different things) avoid consuming gluten due to the damage it does to their intestines in those with Celiac and the discomfort and intestinal distress it causes to those with an intolerance. There are many other symptoms of gluten intolerance/Celiac as well, but the GI distress is the most prevalent usually.
For those with Celiac, gluten consumption can ultimately be deadly. Consuming even a little gluten or having cross contamination with gluten containing foods can lead to serious complications from damage to the intestines. In the past 10 years or so there has been a big spike in Celiac diagnosis and gluten intolerance so that is why you are seeing so many gluten free foods, recipes etc. popping up. In fact, I struggle with this issue myself and have a website and a few hubs dedicated to it.