What it means is the PCM sends a 5 volt reference signal to the MAP sensor and the MAP sensor sends a variable voltage signal back to the PCM. When boost pressure is high the voltage signal is high. When boost pressure is low the voltage is low. The PCM uses the boost control solenoid to control the amount of boost pressure that is produce by the turbocharger while at the same time using the boost pressure sensor to verify that the boost pressure is correct.
Potential causes for this code to set are:
Boost pressure leak
Faulty boost sensor "A"
Faulty turbocharger
Faulty PCM Wiring problem
I'd work my way through the potential failures in this order:
1. Make sure there are no other codes present.
2. Check for boost pressure leaks.
3. Check all wiring and connections to the boost sensor. Make sure the o-ring is on the sensor and check all harness connections for frayed wires, cracked connectors and clean them.
4. Replace the boost sensor.
5. Pull and inspect the turbo.
Also, this video may be helpful:
https://www.youtube.com/watch?v=g_7LgtckNrc