What does 100 200 400 on the film mean?

We bought100,200,400 films in the camera shop, and the numbers represent the sensitivity. Sensitivity is generally expressed by ISO value. With the increase of this value, the sensitivity of the film to light increases, so that it can be shot in different light. Movies like ISO 100 are most suitable for outdoor shooting on sunny days, while ISO400 movies can be shot indoors or in places with poor light, such as early morning and dusk.