What does the mechanic mean when he says a code is "lean running condition"

I own a 1996 Mazda Protégé with 128,000 miles. I had it in for a check of a sewing machine sound. The mechanic tested it and found, among things, a “lean running condition”.

What does that mean?

(I also have more questions that follow in other posts)

It means that the air/fuel mixture ratio has more air in it than it should for the correct mixture. The opposite of that is a rich mixture, or too much fuel in the mixture. A common way a lean condition can occur is when a air leak happens in the intake or exhaust passages.

If the car idles roughly and sometimes stalls while idling look for a leak in the rubber air intake.

Can you post the actual code? It should be on your copy of the shop order.
Under what specific conditions does the engine make the “sewing machine sound”?

The car does not stall while idling but when idling, sounds noisy if in gear and if in neutral, is quieter. No code was posted on the shop order. I have not tested out when the sewing machine sound occurs. I will do that today when I am out and about doing errands. I am making a list of followup questions for my mechanic and will include this one.

How does a lean running condition affect mileage? fuel emissions? safety? Mileage seems to be ok.