Answer:
It's the new standards-based unit short for Mega-Binary-Byte (MiB). It's used because the base 2 representation misuses the terms mega and kilo. The base 2 kilobyte is not 1000 bytes as expected but 1024.
We usually take that for granted and say - OK, accepted. However as an SI prefix we need to use the terms correctly. So instead of 1024 bytes being a kilobyte, it's now called a kibibyte.
This is taking a while to adopt, because we are used to saying kilobyte and megabyte, plus they are way easier on the tongue! however eventually, when referring to a datasize of 1024 bytes we will call it a kibibyte. Kilobyte will be reserved for 1000 and megabyte for 1000000 bytes.