Air pruning is where roots that are exposed to air in a low humidity environment turn off that growth and grow more actively in the moist environment. In one example, plants are grown in openwork containers, like mesh or open weave. When roots reach the limits of the container, they turn off and stop, rather than circling and becoming root bound.
There are air pruning pots you can buy. The goal is to provide a plant with a dense root mass, full of carbohydrates, and ready to burst forth when removed from the pot and planted. Buying plants in air pots helps insure they aren't root bound and crippled. Nurseries like them because they can take up minimal space without causing root binding. It's really a way to keep a plant healthy in a pot that would normally be too small. Or just to provide a healthy well-aerated soil.
Notice how this commercial product has perforated sides and a mesh bottom that is held off the ground, thus air pruning. It's pretty hard to over-water a plant in these. Grow bags that aren't too dense will air prune. Roots reach the arid outer walls and shut down before circling. Keep the grow bag off ground contact for best results.
Once you see the principle, you can come up with all sorts of ways to accomplish it. You can buy Rigid Polyethylene Garden's Drainage Mesh Hole Screens from Amazon. You can use dimpled rigid poly wall drain from Lowes, but you have to drill out the center of the dimples.
https://air-pot.com/nursery/
