On June 15, after over thirty hours of deliberations and several requests for read back of testimony, the Cosby jury indicated that it was deadlocked. Matthew Galluzzo, a criminal defense attorney and former Manhattan sex crimes prosecutor, tweeted the night before that he expected the jury to hang (meaning, to be deadlocked). The judge told the jury to continue deliberating (this is called "giving an Allen charge") and denied a defense motion for a mistrial as being premature. However, Mr. Galluzzo expects the jury to be unable to reach a verdict.
In an American criminal case, the jury's decision as to a particular charge must be unanimous, meaning that all twelve jurors must agree as to the verdict (guilty or not guilty) for a specific charge. A jury can convict unanimously on some charges and acquit unanimously on other charges in the same trial (and that happens quite frequently). A jury indicates to the court and judge that it has reached a verdict by sending out a written note that they have reached a verdict, and the jury emerges to announce the verdict in the courtroom.
Sometimes, as is the case in Cosby's trial, a jury sends out a note indicating that the jury cannot agree as to a particular charge (or any charges). Typically, a judge will then tell the jury that they should continue to deliberate. There is a very specific text that judges are usually required to read back to their deadlocked juries, and this text is typically referred to as an "Allen charge" (based upon a Supreme Court case with that name). An Allen charge is meant to encourage the jury to continue to try to reach a unanimous verdict, and reminds the jurors of the importance of arriving at a conclusion in light of the interests of the parties and the time and resources already spent trying the case.