As with most legal systems, our legal system in the United States is not infallible. This means that sometimes there are people who are convicted of a crime based on evidence who still were not the person who actually committed a crime. This is part of the reason that the United States has a system of appeals that makes it so people can get a second chance.
People who believe that they are being convicted wrongly can make an appeal on the decision that was made. During the criminal appeals process a person can present flaws in the evidence that was presented or the way that an original trial was held to make it so that the decision might be changed. Getting help with this process is critical to making sure you get the most out of an appeal.