Means you aren't getting enough fuel for how much air you are getting.
And you know by taking an Air/Fuel ratio reading and seeing what it says. An audible way is if you hear detonation it could be due to a lean condition.
lean mixture:
A mixture of air and fuel in which there is more air and less fuel
More air + same fuel = higher A/F ratio = leaner
rich mixture:
A mixture of air and fuel in which there is less air and more fuel.. A slightly rich mixture generates the maximum achievable engine power, an overly rich mixture causes spark plug fouling, loss of engine power, and excessive fuel consumption. Any rich mixture increases exhaust emissions
In a spark-ignition engine, the ideal air/fuel mixture ratio of 14.7:1, which must be maintained on engines with dual-bed and three-way catalytic converters.
There are many variables that could cause 1 or 2 of these conditions