The decimal notation of the number twelve is 12:
the digits are ordered from most to least significant.
We could as well reverse the order and write 21, only
that's not the choice that history made for us.
In CPU-speak, our writing style is called big-endian: the digit with the biggest weight comes first. The opposite order is called little-endian.
While big-endian notation has the advantage of being more familiar to us humans, little-endian has some performance advantages (well, on old computers anyway, not sure if it makes much of a difference today.) For this reason Intel chose little endianness for its CPUs. However, MIDI files are big-endian, meaning that every time we read a value from a MIDI file, we need to reverse the digit order.
Since endianness involves reverses digits, this raises the question "What constitutes a digit?"
Endianness works on the level of bytes, i.e., a digit
corresponds to a single byte. Consider the 16-bit number 0x1234,
which in big-endian order would be stored as 0x12 0x34.
To determine the little-endian order, we reverse
the bytes, resulting in 0x34 0x12.