The concept is pretty simple. It’s like the top program, but instead of CPU and memory usage, it monitors disk I/O.
If you have a system that is extremely slow to respond, but can’t seem to figure out what is going on, give iotop a try. You’ll probably have to install it, as I’ve never found a system with iotop installed by default, but it should be in the software repository of just about every Linux distro.