If you Google for “io usage”, you’ll find thousands of articles from many other hosting providers who explain the concept. It’s quite a common system in the world of web hosting, and almost all hosting providers have disk IO limits.
In short, disk IO is the amount of MB per second the scripts of your account are reading and writing from the underlying storage. If you have scripts which continuously read or write many files, or big files, on your account, that would lead to high IO usage.
You might see some benefits from using Cloudflare, or installing a good caching plugin on your website. Both of those options reduce the amount of computation that needs to be done on your website, and might reduce IO usage as well.