It means you will have to seek a waiver for the joint filing requirement. You are allowed to divorce. You will just have to provide significant proof that the marriage was bona fide / genuine when it was entered into. You will also need to attend an interview. It is best that you hire an attorney to make sure this process is done correctly. If it is done incorrectly or if you the petition to remove conditions is not granted, then you will be put in removal proceedings.
Main point- you can divorce if you wish. The government just needs to be convinced that the marriage was geunine at the begining and not entered into just for the immigration benefit even if soured later.
You may call me for a free consultation at (206) 724-1940, any day of the week.
If you get divorced prior to filing your i-751, you can still apply, but you will have to request a waiver from the joint filing requirement. You will have to prove that the marriage was a bona fide marriage and not for the purpose of evading immigration laws. If you are in the process of a divorce and it is not final, the process is more involved. Either way, when the time comes to start the process, you should consult with an immigration attorney about the process.