New DM and I don't understand challenge rating?

Hello all, thanks for reading!

The title kinda says it. I just DM'd my first campaign(probably should've spent more time figuring this out first in hindsight) and I think battles were either too hard or too easy(mostly) with not much in between.

I've tried looking it up to understand and have even watched YT but I really don't understand why I'm having such a hard time grasping the concept of challenge rating.

Can someone help if they have an easy way to explain?