So, imagine you're building a really cool robot with lots of different parts. One of the parts is the boot system, which is like the robot's shoes. But instead of just one kind of shoe, you have lots of different options.
Now, you want your robot to be able to do lots of different things, right? Well, it's the same with the boot system. Depending on what you want your robot (or computer, in this case) to do, you might want a different kind of shoe/boot system.
So, instead of having just one big boot system that does everything, you use a modular boot system. This means you have lots of different parts that can fit together and make a boot system that's perfect for what you need.
Each part might do something different – like one might be really good at handling security, while another is great at starting up your computer quickly. When you put them all together, you get a boot system that's custom-designed for your needs.
That's basically what a modular boot system is – it's a bunch of different boot parts that you can mix and match to make something that works perfectly for you.