The habitable zone is the distance from a star where one can have liquid water on the surface of a planet. If a planet is too close to its parent star, it will be too hot and water would have evaporated. If a planet is too far from a star it is too cold and water is frozen.
Stars come in a wide variety of sizes, masses and temperatures. Stars that are smaller, cooler and lower mass than the Sun (M-dwarfs) have their habitable zone much closer to the star than the Sun (G-dwarf). Stars that are larger, hotter and more massive than the Sun (A-dwarfs) have their habitable zone much farther out from the star.