98 contour se getting code P1151

check engine light comes on and off took it to autozone it gave code P1151 they couldnt tell me what it was, printout says lack of HO2S21 switch _sensor indicates lean. Could someone please tell me what this means i dont know that much about cars. THANKS!

it means exactly what it says. your air fuel mixture is too lean. (i.e. too much air and not enough fuel)