I could go through a list of hundreds of codes and explain what each mean, but sadly for Rovers and most other vehicles, the codes do not always mean a rock solid answer to the issue. For instance on the D2 you could get the code for a speed sensor issue (don't recall the code off the top of my head). This is normally an indication of either a dirty ABS sensor or dirty Mass Air Flow Sensor. But it can also just be a dirty air filter. Notice how it has nothing to do with speed.
Here is a very good online resource for getting what the code is technically, but remember when asking please always remember we know nothing about your Rover, so Mileage, any work that has been done in the last month or so, last fluid changes, plugs/wires, filters, anything that might help provide the answers. Please post with your question to better assist you.
http://www.eobd2.eu/DTCL.asp