(MegaBytes per SECond, MegaBits per SECond) One million bytes or one million bits per second depending on whether the B is upper case or lower case. However, using "B" for byte and "b" for bit is not always followed and often misprinted. See space/time.

Upper Case "B" for Bytes
MB/sec, MBytes/sec, MB/s and MBps means megabytes per second and is used for disk and tape transfer ratings.

Lower Case "b" for Bits
Mb/sec, Mbits/sec, Mb/s or Mbps means megabits per second and is used for transmission speeds in a network or in internal circuits.