Ah, sure. I think a good way to phrase it is to say they lower the precision. That's basically what they do, convert the high precision numbers to lower precision fomats. That makes the computations easier/faster and the files smaller.
And it also doesn't apply to text, audio and images. As far as I know quantization is mainly used with LLMs. It's also possible with images and audio, but generally they don't do that. As far as I remember it leads to degradation and distortions pretty fast. There are other methods like pruning used with generative image models. That brings down their size substantially.