"Kicking back" means "relaxing". For example:

Just kick back and make yourselfat home.

It's nice to kick back every once in a while, isn't it?

People usually "kick back" by sitting on a comfortable chair and:

talking with friends

watching TV

reading

snacking

There are some activities that are relaxing but wouldn't be described as "kicking back", like doing exercise, getting a massage, or meditating.

"Kick back" is a casual expression.