The function of the Meggy Jr Library is to provide a software interface to the Meggy Jr RGB hardware. One of the things that it does is to allocate a chunk of the AVR microcontroller’s RAM to act as Display Memory, analogous to the video memory in a desktop computer system. The Display Memory is a sizeable array of data that fully describes the state of all 200 LEDs on the Meggy Jr RGB. The LED display is constantly redrawn at a rate of 150 times per second, fully reading out the contents of that Display Memory and using it to control the LEDs.

The Meggy Jr Library provides interface calls to directly set and read values in the Display Memory. While this can give you great control over the LEDs, it can also involve unwieldy data manipulation. In most cases the Meggy Jr Simplified Library is a much better place to start.