Do you know how when you're playing a game on your tablet or watching a video on YouTube, sometimes it takes a long time to load? That's because the tablet or computer is trying to get information from the internet or from its own storage, and it's not always ready to give you everything you want right away.
In computer lingo, this is called an "I/O bound" situation, because the device is waiting for input or output (I/O) to happen. When something is "bound," that means it's stuck or not able to move forward until a certain condition is met.
So when a computer program is I/O bound, that means it's waiting for something to happen before it can continue doing its work. It might be waiting for data to be read from a hard drive or network connection, or for a user to click a button or type something in.
Being I/O bound isn't always bad- sometimes it's just part of normal computer processing. But it can be frustrating when you want to use your device quickly, and it's taking a long time to respond. So programmers try to find ways to reduce I/O bound situations and speed up processing, to make our devices work as smoothly as possible.