A biographical dictionary is like a big book of stories about important people. It's like a big list of short stories about people who did really cool things in their lives.
Each story tells you about a different person, like what they did for a living, where they lived, and why they were important. It's kind of like a history book, but instead of just learning about events, you learn about the people who made those events happen.
So if you want to learn about famous scientists, for example, you can look them up in a biographical dictionary and read a short story about their life and all the cool things they discovered. It's like having a whole library full of interesting people to read about!