The ISO number indicates the camera sensor’s sensitivity to light.
The main disadvantage in increased ISO is the introduction of image noise. Noise could add color artifacts and decrease image sharpness and detail. As technology progresses, higher ISOs can be reached with minimal noise.
The typical ISO range is from 100 to 6400 but many newer DSLRs can go as high as 25600 and some even higher.
Here are samples of how ISO values affect image quality:
Many image processing software like Lightroom have good noise reduction features which also allows for higher ISO settings if needed.