big- and little-endian refer to the order in which bytes are stored in memory. With big-endian architectures the most significant byte is stored first (lower in memory), with little-endian, it's stored last.
e.g. the number 258 as a 2 byte integer is stored in big-endian representation as
00000001 00000010
With little-endian it's
00000010 00000001