The endianess defines the way data is stored in physical memory.
Into the issue
There are two standards:
Both are well-widely used and common.
Little endian requires the bytes with the numbers with lower weight to be placed first in memory.
Big endian requires the bytes with the numbers with higher weight to be placed first in memory.
Other endianness exist (such as middle-endian) but are mostly very old, outdated and non-standard.
Endianness used to take to think about storing numbers, but since Unicode's rising popularity, the endianness problem is now into text too, since in Unicode one single character might require more than just one byte to represent it.