A witch records all of her/his spells, rituals and recipes in a Grimoire. It is a spell book filled with information, hexes and spells.
Bonnie's grimoire was passed down from generation to generation and Bonnie uses her ancestor's Grimoire to learn her craft. Jonas Martin had an obsession with Grimoires and collected them. This was important in finding the spell to channel the power of the dead Witches which will possibly lead to the end of Klaus and to find and save his daughter. After Jonas and Luka died, Bonnie packed up their grimoires and presumably kept from in order to kill Klaus. Bonnie wanted to harness the power of all of the dead witches to kill Klaus but if a witch tries to access that much power it would kill them. When Bonnie finally harnessed the power of her ancestors she nearly brought Klaus to the brink of death, but Klaus told Elijah if he rescued him he would show Elijah where he hid their family.