VRLA stands for Valve Regulated Lead Acid battery. These batteries are also called sealed lead acid batteries. They are "sealed" to prevent the escape of the electrolyte but are "valve regulated" to prevent the buildup of gases in the case due to overcharging. By comparison, automobile batteries are generally referred to as "flooded" and are not sealed but are open to the atmosphere and will require the periodic replenishment of the electrolyte.
VRLA batteries operate as a starved electrolyte system in which the quantity of electrolyte is limited to the amount that either is absorbed in the plates or wets the fibers in the separator. The resulting system allows gases produced during overcharge to diffuse from one plate to the other and to be recombined back in the electrolyte. This recombination creates a closed system, thus reducing the release of gases into the atmosphere under normal overcharge conditions. Since the electrolyte is recycled, there is no need to add water to the batteries. This reduced maintenance leads to a longer-life battery in a clean and compact package that can be installed when flooded cell batteries are impractical.
(Source: "Valve Regulated Lead-Acid Batteries", Office of Transportation Technologies, US Department of Energy.)