A science journal is a kind of science diary. It's a place to record what you have done in science over a long period of time. It's also a place to jot down notes about what you would like to do in the future. You can also write down day-to-day observations that you are curious about. These observations might lead you to ask yourself questions about nature that you would like to answer.
Your journal might be made up of a collection of loose-leaf pages from different investigations. These pages might be three-hole punched so you can keep them in a binder. Your teacher will tell you how to put together a science journal and what should go in it.