"Network monitoring" means watching all the things that are connected to the internet to make sure they are doing what they are supposed to. Just like how your mom might watch you and your siblings to make sure nobody is doing anything dangerous or naughty.
Imagine you have a big toy box full of your favorite toys. The toy box is your network, and each toy is a different device that is connected to the internet, like your tablet, your parents' phones, and your family's laptop. Network monitoring is when someone, like a special computer or a grown-up, is always keeping an eye on that toy box to make sure all your toys are working properly, nobody is using them too much, and nobody is breaking them.
Just like how your mom might check in on you and your siblings while you're playing, network monitoring can help keep everything safe and organized. For example, if a device in your network starts using too much internet (like if your brother starts downloading a very big game on the family laptop), network monitoring can send an alert to your parents or a computer administrator to let them know something unusual is happening. That way, they can take action to stop the download or fix the issue.
So, just like how your mom keeps an eye on you and your toys, "network monitoring" is like having a grown-up watching over your internet-connected devices to make sure everything is working smoothly and safely.