Okay kiddo, so a koncovka is a word that is added to the end of another word to help tell us more information about it. It's kind of like when you put a sticker on your toy to show that it belongs to you.
For example, let's think about the word "píšeš" which means "you write" in Czech. The "š" at the end is the koncovka, and it lets us know who is doing the writing - "you" in this case.
Koncovky can also tell us things like when something happened, like with the word "přijel" which means "he arrived" in Czech. The "l" at the end is the koncovka, and it tells us that this happened in the past.
So we use koncovky to help us understand more about the words we are using. They might seem a little tricky at first, but with some practice, you'll be a koncovka pro in no time!