LED means Light Emitting Diode. It is a small light bulb. This differs from incandescent light bulbs in a few key areas, it does not have a filament, which makes it more efficient and have a longer life span.
The individual LEDs are grouped together in what is known as a pixel. The number of LEDs in each pixel varies. There could be one LED or there could be 4 LEDs in one pixel. The matrix of an LED display is made up of the number of pixels high and the number of pixels wide. The matrix is what determines that physical size of the sign. If an LED sign is 24 pixels tall and 96 pixels wide, the matrix is 24×96.