What does an "I/O Task" mean?
Reading / writing from disk.

Reading / waiting for keyboard / mouse input.

Reading / waiting for network data.

Printing a page.

Basically, as the text says, an I/O task is anything which the CPU can't perform on its own, and has to rely on other components. Usually this involves waiting a long time, compared to the CPU speed, so it's better to switch to another task while waiting. 

In this way, even if the browser is waiting for the server to respond, the other running tasks can move on (unless they also need to wait for I/O, of course).