A more thorough explanation:

An open verdict is a type of verdict given by a coroner's jury when they cannot determine the cause of death or the identity of the person responsible for the death. The verdict states that the person "came to his death by means to the jury unknown" or "came to his death at the hands of a person or persons to the jury unknown."

For example, if a person is found dead with no clear evidence of how they died or who caused their death, the coroner's jury may give an open verdict. This means that the case remains unsolved and the cause of death remains unknown.