You know how some languages write left-to-right, and some rught-to-left? Endianness is that, for numbers.
Or another analogy is dates: 2025/12/31 is big endian, 31/12/2025 is little endian. And 12/31/2025 is middle endian. Which makes no sense at all because the middle is, by definition, not an end.
Rijndael is pronounced AES.