If your I-485 Adjustment of Status application is denied, it means USCIS has decided not to grant you lawful permanent resident status (a green card). The denial notice will explain the reasons, which may include ineligibility, missing evidence, inadmissibility issues (such as unlawful entry or criminal history), or failure to maintain status. Importantly, you generally cannot appeal a denied I-485, but you may have options such as filing a motion to reopen or reconsider with USCIS if you believe the decision was an error or submitting a new application if you become eligible later.