That's like going back to uni class CS101, anyway

Endianness is ordering the bytes (or bits) in the memory, there two notations, big-endian (most significant bit) or or little-endian the (least significant bit).
So in simple words little-endian means saving the bits in reverse order.

Bitcoin use litte-endian notation.

Bitcoin is using SHA-256 algorithm for Block hashing -> it produces hash in little-endian (leading x number zeros)

For example ,this is the latest block Block #522238, below is hash and it is shown in big-endian notation:

and in little-endian notation (Swap the bytes):
Please ignore any typo as I typed in reverse manually

Now why litte-endian for blockchain its arguable, there are computational advantages in using little-endian but on the bigger picture there is no difference.