"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
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.