[200hp/tonne]

^ what he said.

Simply put, a turbo is only capable of so much boost if it is moving a fixed amount of air (by mass). That is the pressure ratio vs mass flow portion of the compressor map.
Compressor maps also have other useful information in them, like
1. for small flow numbers, turbo compressors have a boost boundary, above which the turbo surges and gets damaged (surge line)
2. Efficiency islands: at your boost vs flow operating point, it helps you estimate what the efficiency of the compressor is, which helps to calculate theoretically how hot your air coming out of compressor will be
3. Turbo speed numbers: estimates turbo shaft speed at operating point. you can use turbo speed sensors to control your turbo or your engine mapping, it is a nice fast easy method of ECU control logic
4. Mass flow limits (choking): direct indication of max power limit of a turbo on gasoline