They were referring to almost all talk about the war, both abroad and domestically. For example, people who worked in war related industries would be strongly discouraged from talking about their jobs outside of work, and families would be told not to talk about those they knew in the war - where they were fighting, who they were with etc. This was all to avoid anything being overheard by Axis spies and reported back.

Talk about troop movements, plans for attack, or any information about the British war effort that could be harmful if discovered by the Germans. While the censors blocked most information like this from letter home by soldiers, some slipped by.