Form I-751 is intended for conditional residents whose status was obtained through marriage to a U.S. citizen. Not everyone who married a U.S. citizen becomes a conditional resident -- only those whose marriage was less than two years old when the immigrant was either approved for residence or entered the U.S. with an immigrant visa.
The status is made "conditional" because of concern that such a new marriage might be fraudulent -- that is, entered into for the purpose of evading immigration laws of the United States. The two-year expiration date on conditional residence gives U.S. Citizenship and Immigration Services (USCIS) a second chance at seeing whether the marriage is the real thing and whether the immigrating spouse deserves a U.S. green card (permanent residence).
There is some confusion about when you need to file Form I-751. You are supposed to submit it within the 90 days prior to the two-year anniversary of having been approved for conditional residence. Some couples, however, mistake the second anniversary date as the one referring to their date of marriage. That date is irrelevant in this context.
Look on your conditional resident card, where you'll find your approval date. Start counting from that date forward two years, and then subtract 90 days to figure out the earliest date upon which you can file Form I-751. Count the days carefully! Don't just think, "30 days is about a month, I'll subtract three months," or you might get it wrong. You want to make sure you start early enough to gather the necessary paperwork.
If you file Form I-751 too early, USCIS will return your application to you and you'll have to refile it. This might take several weeks, which will delay your ultimate approval.
If you don't file your I-751 before your conditional resident status expires, your status will terminate automatically and you can be removed from the United States (deported).
If the failure to file on time is no fault of your own, you can, if you hurry, submit Form I-751 with an explanation and proof that your late filing is for valid reasons that were beyond your control. USCIS has the discretion to approve or deny your late filing. But you'll need to act before your case is transferred to immigration court, so contact an immigration lawyer immediately if you're in this situation.
Filing Form I-751 is more complicated that you might think, requiring careful preparation and assembly of supporting documents. In order to have the best chances of getting it done on time and receiving USCIS approval, it is recommended that you get help from an experienced and competent immigration attorney. Hiring a lawyer will help ensure you avoid legal snags that can pose serious immigration problems, so it's well worth the additional cost.