The phrase "empty bladder" means to urinate or release the liquid waste that accumulates in your bladder. So, it refers to the act of making sure your bladder is completely empty by getting rid of urine. Full definition
He could never fully empty his bladder and this caused many accidents and a very slow learning process. (thebabycorner.com)
The general guidelines for all yoga practices apply: practice on a light stomach, empty the bladder beforehand, and stay within your comfort zone. (yogainternational.com)
It also plays a song to ensure a child stays on the potty long enough to relax and fully empty her bladder. (howtoadult.com)