It's a phrase used to describe a specific type of notebook, for example:
* Mon cahier de français: My French notebook
* Mon cahier de maths: My math notebook
* Mon cahier de notes: My notebook for notes
* Mon cahier de dessins: My drawing notebook
The word "cahier" is the French word for "notebook" or "exercise book."
Let me know if you have any other French phrases you'd like help understanding!