What does P0129 barometric pressure too low mean?

The Manifold Absolute Pressure (MAP) sensor responds to changes in the intake manifold pressure (vacuum). 5 Volts is supplied to the sensor from the PCM (powertrain control module). Inside the MAP sensor is a resistor that moves in relation to manifold pressure. The resistor varies the voltage between about 1 volt to 4.5 volts (depending on engine load) and that voltage signal is returned to the PCM to indicate manifold pressure (vacuum). This signal is essential for the PCM to determine fuel delivery. A P0107 DTC is set when the PCM sees that the MAP signal voltage is less than .25 volts which is too low.