A Solar regulator aka charge controller is used with a stand-alone or off-grid solar power system or in a hybrid grid connect system with batteries, in a grid-connect solar power system with no batteries there is no need for a charge controller.
A solar regulator takes the form of a smallish box-shaped object with some solid state electronics inside and is installed between the solar panels and the deep cycle batteries that are to be charged by the panels. The role of the charge control unit is to monitor and control the amount of charge being sent to the battery bank and ensure overcharging does not occur; a regulator can also act as a power source on the circuit before the batteries allowing for the powering of some electrical appliances while the batteries are being charged. Modern solar regulators are highly efficient in their operation outperforming most 3 stage mains battery chargers.
To find the correct size regulator required, add up the amp ratings of all the solar panels in the array, this will show the minimum size charge controller needed for the system. It's not a bad idea to think of the future when purchasing a regulator as you may want to extend the size of your off-grid system sometime in the future so buying a larger regulator can save some money down the track.
Some charge controllers have a low voltage disconnect system built in (LVD). Battery life can be shortened if charge voltage drops below a certain level frequently, the LVD system prevents this from happening by monitoring battery power levels and automatically disconnecting the power supply should the level reach a critically low level, an excellent way to protect the life and health of a deep cycle battery bank and save you some money.
No maintenance is required for solar regulators except for maybe checking the wiring for corrosion which is standard on DC circuitry. A charge controller should be mounted in a cool dry place out of the weather and of course out of the reach of curious young hands. It is also good practice to have a fuse between the solar panels and the regulator to reduce the risk of damage and fire risk.