Sure, I'd love to. It's more ELI10 that ELI5 tho...

It's very likely that you or your brother played an adventure game or an RPG on your computer. When you get lost in such games, or simply want to know where your character that your playing needs to go, you open up a digital map. On that map you usually get all the information you might need - what is your current location, where your active quests are, maybe even different parts of the world, if it's divided into 'zones'.

Such interactive maps are a great example of what GIS does. The town or city you live in, usually uses a similar interactive map. Instead of active quest, their system might show things like parks, points of interest like turist spots or parking spaces. It might also show how many people live in what part of the city, their average age and income.

Beside your local municipalty, other companies or organization also use GIS. Their systems might show other (spatial) data that interest them.

Fire department might have a system that shows historic data - where they've had most fires, what the current situation is and where their units are dispatched at the moment.

Your Internet provider might have a map of their network and any issues along it. Maybe even overlay of property lines, so that they know who to contact when they'd like to expand their network and put new optical cables in the ground.

The system that collects and shows similar data as mentioned above is usually referred to as a Geographic information system. In it's most basic definition it is a system for collecting, storing and displaying spatial data.