Byte ordering specifies the order in which a sequence of bytes is stored. In Little Endian ordering, the least significant byte is first. In Big Endian ordering, the most significant byte is first.
Intel processor-based systems use a different internal data representation (Little Endian) than Motorola processor-based systems (Big Endian), where each representation is optimized for the specific internal architecture and processor being used. This means that for certain 16-bit words, the two bytes might have to be swapped for the information to be exchanged between different devices.
In a DICOM message, data can be communicated using either ordering method (little or big). While all DICOM-compliant devices support the Implicit VR Little Endian Transfer Syntax, it makes sense to encode the information using the byte order that is native to the machine. This eliminates the need to swap the bytes first, then transmit the information, and then swap them back on receipt of a response.