Endianess is how bytes are organized in a multi byte value , it presents the new order of bytes when every part that forms the value is put into a digital word . Bitcoin uses the little Endian to serialize almost all the data with it, and that has been always considered as an error by the community but is IMO an optimization(*) .
And since i mentionned the little Endian , people should know that there's two types , Little and Big Endian (used to serialize network adresses ) .
For a brief explanation let's take for example this hexadecimal "96385214" , if we are using the little Endian it'll be stored in the computer's memory as following : "14 52 38 96" -> we take the byte at the end (called little end) which is 14 and place it in the beginning , and keep proceeding from right to left  .
As for : Why is little Endian  used in blockchain , well it's because the dev team chose to , (*) -> and also because the majority of new generation computers use little Endian so that's why i consider it as an optimization .