What does the trouble code mean

I have a 91 lebaron convertable v.6. It has been parked for about 4 years and when we started it up it ran fine for a few days then it started acting like it was out of gas but when you restart it it runs fine for a few minutes and then chugs like it is out of gas. I pulled the fault codes and the code that pulls up is 51 Lean air/fuel mixturehas been indicated by an abnormallyrich correction factor. What does this mean and what do I do.