What do this mean code p 0171 fuel trim too lean

The fuel / air mixture constantly switches between too lean and too rich a couple of times per second, then the first oxygen sensor measures the amount of unburned oxygen in the exhaust system. It sets that code if there's too much oxygen and not enough fuel. Common causes of too much air include a vacuum leak, low fuel pressure, varnish buildup in the injectors, and a dirty mass air flow sensor.