Endianness is the order in which a multi-byte number is stored in memory.
Big-endian stores the most significant byte first (at the lowest address), while little-endian stores the least significant byte first.
Storing hex 0x1234, big-endian places the bytes as 12 34, and little-endian as 34 12.
x86 CPUs are little-endian, while network protocols usually use big-endian (network byte order).
When two systems of different endianness exchange binary data, values come out reversed unless the byte order is matched.