Circular breathing is a technique used by woodwind and brass players to create a theoretically endless stream of air being able to play theoretically forever. The technique works in the way that you fill your cheeks with air before your lungs are almost empty and quickly inhale while using that air to keep the stream that goes into the instrument flowing.
While this allows theoretically to play forever it comes with a few things to consider. It is technically quite demanding and not all players can do it. Additionally for instance flutes who need to keep their mouth/lips/cheeks in a specific position in order to control the sound really struggle doing circular breathing. The most important thing however why this technique can not be used forever is that your players will sooner or later run out of oxygen or have too much oxygen in their system. Most of the time the air that is inhaled and exhaled during playing and that passes through the instrument is not enough or too much to meet the demand of breathing of the player’s bodies. So sooner or later your players need to get the chance to breathe normally. Keep that technique only as an option for special cases, not every day playing.