Proto-Uralic language is a really old language that people think existed a long time ago in a place called Ural. It's like a grandparent language that lots of different languages that people speak today (like Finnish and Hungarian) came from.
Just like how you learn to talk by listening to your parents and copying the sounds they make, back then people probably did the same thing - they listened to their friends and family and started talking like them too. But over time, people moved to different places and started talking differently, and their languages changed too.
That's why we have so many different languages in the world today! But linguists (people who study languages) think that if they study all the languages that still exist and try to find similarities, they might be able to figure out what Proto-Uralic sounded like back then. It's like a puzzle - you take little bits of different languages and try to put them together to make a bigger picture of what the original language might have been like.