It means this person told the judge, during her trial, "I did it." People often do this as part of a deal. The judge will agree to reduce their sentence (punishment) if they plead guilty. When evidence is clearly against a person, pleading, "Not guilty." wastes a lot of time, so judges make deals in order to move on to another case.

When a person is charged with a crime, he or she is asked in court to answer the charges with a plea: "How do you plead?" The basic pleas are "innocent" and "guilty". (There are a couple of others, but these cover the basic yes/no response to the charges.) By Lakeisha pleading guilty (ie. admitting that the criminal charges were true), the case didn't need to go to trial and she would have received a sentence as punishment.