In short and simple terms, "to run a bath" means filling up a bathtub with water to take a bath or soak in it.
Sentences with «to run a bath»
To use, simply add to warm running bath water as desired and enjoy. (soapdelinews.com)
I simply run a bath for them like normal, and drop 5 drops of my three favorite essential oils for relaxation into the water. (mindbodygreen.com)
I love doing all these things to relax, I actually run a bath, light some candles and read a few chapters of my book in the bath! (kerrylifeandloves.com)