The meaning is when your mind is steady and your pose is comfortable. It’s time for asana to come perfectly.
Let the mind pure, do not think anything, focus on the breath and chakra inside the body. When you already steady your mind, you can come to asana easier and more comfortable.
Let’s imagine that somebody who cannot forcus on as he/she is practicing yoga, the pose will not good and cannot be balanced.
For me, I sometimes also don’t focus on my pose and don’t let my mind steady when I practice headstand. When I did this asana, I always looked at the mirror to make sure that my neck was straight or not, and I that time couldn’t balance my body and did fall down immediately.
Final statement is that the steadiness of mind must be required to have the best asana when practicing yoga.