The simple answer is that a “Grimoire” is a personal notebook for a Hedge witch (or any other kind of practitioner). It is where you keep your thoughts and notes about what you are doing; where you keep track of experiments or things that you learned.
It can be a diary, a spiral notebook, a fancy journal, or a collection of documents online in a special folder. It might also be a recipe box, depending on what kind of Hedge Witch you are. Or it could all of those things if you are someone who practices across a variety of disciplines.
Some people keep their recipes for poultices and potions; some make note of when the best time is to plant a certain herb or fruit or vegetable; some have protection spells; some have their grandmother’s secret recipe for stuffing.
It is up to you what you keep in your grimoire. Mine have changed over the years, and I suspect yours will, too. Some websites out there will offer you downloadable pages for your grimoire. Some will outline what they think you should have in your grimoire. Make up your own mind about what belongs and what doesn’t. There is NO RIGHT OR WRONG, there is only personal preference about what you might want to record or have as a reference for the future.