Solar power regulators are used to prevent the battery bank connected to your solar panels from becoming overcharged.
Solar panel regulators monitor the flow of power between the solar panels and the connected battery. Some regulators may simply disconnect the flow of power when the battery is in danger of overcharging. Others monitor the flow and, when the battery voltage rises to the regulation point, they regulate the flow to hold the voltage at this point for a period of time. This is called the 'absorption' period, and generally allows the battery to become fully charged. The regulator then controls the charge to keep the battery at close to full charge, also known as the 'float' point.
Solar panel regulators come in different sizes depending on the amp rating of your solar panels. They usually have some kind of display to tell you which stage the regulator is in, which can be simple system of lights or a full LED display.