ISO
ISO stands for the International Organization for Standardization β an organization that sets international standards for all different kinds of measurements. But for our practice and use ISO refers to a film's sensitivity to light, the lower the ISO 100, 200 the more light is needed to expose the film (so 100 and 200 films are best for outdoor daytime use) whereas, high ISO like 400 and 800 are MORE sensitive to light and therefore are best in low lit indoor lighting scenarios. With digital. cameras the imaginary ISO can go as high as 256,000 and while there will be a lot of "noise" in that image, you can capture something that is almost in a pitch black room.
Another great resource for ISO is linked below:
Resource 1 : https://www.phototraces.com/definition-of-iso-in-photography/#ISO_in_Film_Photography