Gluten is a protein found in cereals such as wheat, rye, barley and oats. When gluten is mixed with water, it become stretchable and forms sticky fibers that and when get bonded, it make bubbles making bread and sweets more soft and fluffy.
Speaking of the word "gluten free" we suggest a lack of gluten. In fact, there is no test that is sensitive enough to detect a zero level of gluten. Only foods containing less than 20 parts per million can be labeled "gluten-free". This allows those people who have a high sensitivity to gluten to be able to consume them.