Okay, little one. Have you heard the phrase "little clause" before? Se'if katan is just another name for it. It's a small part of a bigger rulebook that helps Jewish people know how to live their lives by following the Torah.
Think of it like a puzzle with lots of little pieces. Se'if katan is just one of those pieces that helps complete the bigger picture. It might seem small and unimportant on its own, but when you put all the pieces together, it helps Jewish people live their lives in a good and kind way.