Okay, imagine that a department is like a big club or team. Just like how you have a teacher in your class who helps your teacher out, a department secretary is someone who helps the leader of the department, who is called the "chair" of the department.
Their job is to help the department run smoothly by doing lots of different tasks like answering the phone, scheduling meetings, keeping track of important documents, and helping people in the department when they need things.
It's kind of like being the helpful friend who always knows what is going on and is ready to lend a hand. Without a department secretary, things would be much harder for the chair and the rest of the department.