By mixing hundreds of different proteins of the same class constitute Gluten. But mainly gluten is made of two types of proteins :
1) Gliadin - Responsible for the bread to rise when it is baked
2) Glutenin - Provides elasticity to the dough
So Gluten free food means which doesn't contain gluten in it. Basically gluten is found in oats, wheat, barley,etc. Consuming gluten can cause many problems in your body if you suffer from Celiac disease. Small intestine can be damaged. Also it affects the immune system . So it becomes difficult for the body to fight against diseases.