When you read a book, it usually has a lot of words in it. Some books have so many words that they have to be split up into different parts to make it easier to read. These parts are called volumes.
Think of it like a big cup of water. If you put too much water in it, it will overflow and make a mess. But if you split that water into smaller cups, it will be easier to manage and not spill everywhere.
In a bibliography, volume refers to the specific part of a book that you are referencing or using as a source for your own work. So, if you are using a book that has multiple volumes, you will need to specify which volume you are using so that others can find the same information.
For example, let's say you are writing a paper and you use the book "Harry Potter and the Philosopher's Stone" as a source. This book has seven volumes, or parts, so you need to specify which volume you used. Let's say you used information from the second volume. In your bibliography, you would write "Rowling, J.K. (1998). Harry Potter and the Philosopher's Stone (Vol. 2)."
By including the volume number in your bibliography, you are helping others find the exact information that you used, just like how splitting the water into smaller cups makes it easier to manage.