Compostable films are made from bio-polymers, such as starch or polylactic acid (PLA). Just like biodegradable films, they require certain conditions to decompose and under legislation, they must meet certain criteria to be classified as compostable.
Most compostable films are considered compostable at an industrial level, so aren't suitable for home composting like you might think. Whilst they'll still break down in a home compost heap, they'd take longer than in industrial conditions. When they do breakdown, they turn into water and CO2. Whilst the latter might seem like a negative, it often has a neutral effect on the environment. This is because the plant-based materials used to make the films absorbed CO2 in the first place. They also break down a lot quicker than biodegradable plastics.