What is difference between low resolution and high resolution photos?

digital (higher res): more pixels making the same image = larger image and larger file size. Even if the picture is re-sized by software, the file size does not change unless it is cropped or compressed. Compression usually lowers resolution.

b&w film (higher res): more grains of black metallic silver in the same size image. This is primarily accomplished by using slower ISO rated film. Not to be confused with acutance (sharpness due to developer/development).

color film: same as b&w except bms is replaced by dye "spots".