This is a measurement of speed typically used in non-metric countries for transport such as the USA. The United Kingdom also uses this on the roads although officially the metric system has been adopted. Road speed limits are given in miles per hour which is abbreviated as mph or mi/h.
Mach is a speed measurement based on the speed of sound. Since sound travels at different speeds in different conditions, the calculations here are given for 20°C in dry air at sea level. Mach is typically used in aviation and space exploration.