Okay kiddo, so you know how sometimes it takes a looooong time for your phone or computer to load something from the internet? Fast ethernet is a way to speed that up!
Basically, when you want to look at something online, your device sends a message to the internet asking for it. That message travels through something called an ethernet cable, which is kinda like a really long string that connects your device to the internet.
Regular ethernet is like a regular string that moves kinda slowly. But fast ethernet is like a super strong, super fast string that can move a lot of information really quickly. So when you use fast ethernet, your device can talk to the internet and get information back a lot faster than with regular ethernet - which means you can watch videos, play games, and do all sorts of cool things without having to wait so long!