Endianness refers to the choice of how lower and higher byte addresses are used in computer memory. The practice of using lower byte addresses for less significant bytes is little-endian. The opposite practice of using lower byte addresses for more significant bytes is big-endian.
As a more concrete example, say the operation code is 02000284. In a little-endian architecture, it’s stored in memory byte-by-byte with 84 02 00 02, where 84 is at the lowest byte spot and 02 is at the highest byte.
This design choice is mostly arbitrary, as long as people who work with the architecture remain consistent. Many networking protocols are big-endian, while many processor architectures (x86, ARM, RISC-V, Nios II) are little-endian.