Water popping a hardwood floor, is a technique used to richen a stain color that is applied to a raw hardwood floor during refinishing. This is something that has to be done when the desired end result is a darker pigmented color. To help you understand I first need to explain the sanding process. Keep in mind, not all stain colors require water popping.
The sanding process starts by using a course grit sand-paper to flatten and clean the wood floor; then progressively climbs to a finer grit to smooth the wood to finish quality specifications; otherwise the floor will be rough. As you climb to finer grits, the tighter the grain and knapp fibers of the wood become making the wood less penetrable. If a dark stain is applied over a freshly sanded "smooth" floor the pigment cannot penetrate sufficiently to give you the rich dark color sought. The color will be a pale shade because after the stain is wiped on, it comes back off when the "dry" rag is used to absorb excess, and to make even.
It's fair to say water changes the characteristics of wood. Thus by properly applying a denatured alcohol & water mixture to the surface, then blow drying it with high speed blowers, the closed grain of the wood "pops open" while maintaining the smoothness required for a quality finish. It allows the stain to penetrate deeper into the open grain of the wood, resulting in the dark rich color as intended.