A high quality power supply that takes AC (the power in your house, United States) and converts it to 12v DC, a very common and usable power source for powering leds or other projects. With this particular power supply you can simply plug it into the wall and be done.
If you wire a "single" led at a time to this power supply (all in parallel) you can run about 112.5 leds off of this power supply. Here is how we figured this number. If you think of a single led consuming 20mA and the fact that this power supply can offer 16.5A (16,500mA) then you can divide out the numbers. We recommend that you run the power supplies at only 75% of their max capacity for longevity purposes, for your project and leds.
(3,000mA/20mA)*.75 = 112.5 LEDs*
Please note that if you use leds that aren't 20mA this number will change. There are also other ways to wire them up as well that will allow different numbers. Please see the differences in wiring leds up in series vs. parallel. For all practical purposes if you don't have much experience with wiring leds and want to get things lit up quickly just wire them all up as suggested above.
*These numbers are based off of quick calculations, please be sure your power supply can handle your projects. These numbers are meant for demonstration purposes and we take no responsibility with what you do with them.