In healthy lungs, there is a constant flow of mucus(a slimy liquid) over the surfaces of the air passages. This removes debris and bacteria. If you have CF this mucus is excessively thick and sticky due to an altered balance of salt and water in the lungs. CF also affects the digestive system. In a healthy person, the pancreas produces enzymes(chemicals) that pass into the gut. These enzymes break down the fat, carbohydrate and protein in food. If you have CF your pancreas still produces these enzymes but the duct which takes them to the gut is blocked by thick and sticky mucus.
Attribution: BUPA