Frogs exchange gases through their skin and the lining of their mouths. This exchange of gases is referred to as respiration. During the larval stage in frogs development, it lacks functional lungs but they can take in oxygen through gills which are in process of metamorphosis.
The gills take in oxygen when water is being passed over them. Once lungs are mature, they lose their gills and are able to bring in oxygen through lungs.
They rely on lungs to breath when they are active and they need more oxygen than what skin respiration alone can provide.
Although they have oxygen, most of their respiration occurs through the skin.