Endianness refers to how data is ordered in machine language, the simplest, most understandable code that a computer can use.
In computer coding, all data (information) is just a series of numbers, which can be written and / or input in two ways: little-endian and big-endian. Which one is used depends on the data format and the way the computer reads the data.
Say that we have a number like this:
12AB34CD ---> 12 | AB | 34 | CD
The way it's written right now is big-endian, because the bigger number is on the end. (Note that this number is in hexadecimal (base-16) ).
There are two important things to know here: first, "end" here does not mean "the last", but rather "side". In other words, "big-endian" means something like "the big side first", not "the big number is at the finish". Second, in computing numbers are usually grouped into bytes which hexadecimal uses two digits to write out. Each group is treated as a single thing and the digits within do not switch.
To write it in little-endian, we simply switch the order of each group, so it becomes:
12AB34CD ---> CD | 34 | AB | 12
Note that it is the place of each group that changes, and the number does not become DC43BA21.