A dibber is a cylindrical device used to push down semi-solid material.

In gardening, a dibber is about an inch wide, and used to push down soil, to make room for bulbs or seeds. At a push1, the handle of a trowel or one's finger both make perfectly acceptible dibbers.
When hand-rolling a cigarette or joint, a small dibber is used to push down the material to allow a filter or roach to be inserted. Some lighters have a removable flint assembly that can be used as a dibber.
It is entirely possible that 'dibber' is a chiefly British word (as microsoft bookshelf is fond of saying).

1 - hoho.