So, imagine you are playing with Legos and you want to build a really cool tower. But you need some rules to make sure all the pieces fit together properly and your tower doesn't fall over.
Well, the European Telecommunications Standards Institute (ETS) is a group of really smart people who make rules for how all the different parts of the internet and phone systems in Europe work together. They make sure that if you call your friend in another country or try to send an email, everything works smoothly and your message gets to the right place.
Just like how you need to follow the instructions when you build with Legos, companies that make phones and other tech gadgets need to follow the rules set by ETS so everything works together properly. This way, you can play games on your phone, watch funny videos online, and talk to your friends all across Europe without any problems!