Basically MB/GB means the prefixes M (10^6) and G (10^9) are counted in decimal. MiB/GiB means the prefixes are counted in binary - Mi (2^20), Gi (2^30).
See the article on Wikipedia: Binary prefix
Vendors specify capacities (of hard drives for example) using GB/MB which results in a higher value compared to GiB/MiB. However, Windows (and possibly other OSes) will usually only tell you the GiB/MiB value, despite the fact that it writes GB/MB as unit behind the value