Endianness is the ordering convention that two parties that wish to exchange information will use to send and receive this information when they need to cut the information down to pieces.
Most modern computer processors agree on bit ordering "inside" individual bytes (this was not always the case). This means that any single-byte value will be read the same on almost any computer one may send it to. Integers are usually stored as sequences of bytes, so that the encoded value can be obtained by simple concatenation. The two most common of them are:
increasing numeric significance with increasing memory addresses, known as little-endian, and
its opposite, most-significant byte first, called big-endian.