Constant voltage power supplies are also know as LED drivers. They provide a stable 12V DC supply to LED bulbs and flexible LED strips, ensuring the life isn't reduced by using an incorrect supply.
This LED driver is a rated at a maximum of 48W (4A). It also comes with a connector to allow bare wires to be connected without the need to modify the driver's connector. All or laptop style drivers come with a fused UK 3 pin plug.