Mould spores are everywhere. They can be found outdoors where there they are primarily associated with certain plants and soil. They are also found in foods such as mushrooms, fermentatious yeast, and ripening cheeses (Brie, Roquefort). And because mould spores are everywhere, they also find their way indoors through windows, doors, shoes and clothing.
Mould spores that are indoors are not a problem on their own, but if they are given the correct conditions and combination of temperature, light, food and especially water, they begin to grow and potentially create toxins that are dangerous for pets and humans to be around.