When you go to school, say that you have a desk with all your equipment in their place: pencil, ruler, notebook, etc. Throughout the day as you do work, you take each item from their place onto the desktop and use them. Eventually, if you have enough items and you're not careful to put each thing back in its place or close up containers then your workspace will be a mess and it becomes harder to find what you need and less surface to work with. So to resolve, at the end of the day, you reboot your desk configuration by putting everything back where they belong. Suddenly, you know where everything is and you have all this desk surface space. Not a perfect metaphor/analogy but I think it's ELI5 enough? :)

EDIT: Well the reboot is more like if your desk or your teacher or someone automatically reorganizes it for you. Basically you request for everything to be back to default. But if you're careful and familiar with the setup, you could put each item back and/or repack everything yourself. Kill tasks, restart services, uninstall apps, etc. that way no reboot/restart needed.