A Mention Hearing at Crown Court is a hearing to bring issues in the case to the attention of the Judge, hopefully the same Judge at trial.
A listing can be made for various reasons, i.e. insufficient disclosure from prosecution etc.
Under the previous legal system it would be usual to have a mention to see how trial preparation is going and the Judge could make directions for disclosure to be served by a certain date or for a defence case statement to be served by a certain date if not already ordered.
Under new provisions for Crown Court prosecution mention hearings are less likely to be made due to the administration of simple, speedy justice.