Endianness is the way that the bytes in a word are ordered when stored in memory. A big-endian word is stored as the most significant byte first, and a little-endian word is stored as the least significant byte first. If we visualize a word as a series of bits as seen in the diagram below, the most significant bit is on the left and the least significant bit is on the right. This is also how they are stored in memory and matches how they are stored in the processor.