No obligation (don't have to)

Don't have to means there's no obligation to do or be something. You can if you want, but it's not necessary. If something is free, you don't have to pay. If something is optional, you don't have to do it.

To help you remember, here are some lyrics from the Prince song Kiss:

You don't have to be rich
To be my girl
You don't have to be cool
To rule my world

Of course, it helps to sing these in a really high voice. I would be happy to sing them with you. But you don't have to sing them if you don't want to...