1998 Pontiac Bonneville Code P0171

Engine Mechanical problem
1998 Pontiac Bonneville 6 cyl Automatic

Hi. Just recently, my engine light went on, and I took it to Advance Auto where they hooked up to the engine computer. They got the code P0171. I looked the code up here on your website, and was wondering what "System too lean (Bank 1)" meant? If you could help explain that, I would much appreciate it so that if I take it to a mechanic, I can tell if they're bullshitting me or not! Thanks.