Your own record of how your thinking changed over the semester.
A journal is chosen, not accumulated. A handful of entries written close to the work, while you still remember which reading you doubted and why, beats a folder containing everything you ever handed in.
Start with Chemistry Journal for what this is and why we bother, then write Your First Entry in the first week. What a Strong Entry Looks Like shows you the standard, and Journal Checklist is what I check against at the end.