Okay kiddo, let me explain it to you in a way that's easy to understand.
So, D-Day was a very important day during World War II when the Allied forces invaded Normandy, France to try and win the war against the Germans. The Daily Telegraph was a newspaper that people would read every day to find out what's going on in the world, including updates about the war.
Now, during the war, it was very important for the Allies to keep their plans and strategy a secret from the Germans. So, sometimes they would use secret codes and messages to communicate with each other. One way they did this was through the Daily Telegraph crossword puzzle.
The people who made the crossword would sometimes use secret messages and codes, and only people who knew what to look for could figure it out. This way, the Allies could share important information without the Germans knowing what they were talking about.
But, the Germans were smart too and tried to figure out the codes. So, the Allies came up with something called a security alarm. This was a way to warn everyone quickly if they thought the Germans were getting close to breaking the code. It was like a big red button that they could press to say "hey, we need to be extra careful right now!"
So, that's the D-Day Daily Telegraph Crossword Security Alarm in a nutshell - it was a way for the Allies to communicate secretly and keep their plans safe during the war.