There are two different types - rusks and biscuits.
Rusks are generally in stick, cylindrical form - they're hard and NOT meant to be eaten - more for them to just grind their teeth on to soothe the teething pain.
Biscuits or wafers are thin - can be eaten like normal biscuits.
However, whichever it is, they should NOT be given to LO when it breaks apart. Most biscuit packs would also give a warning behind that it should not be consumed if broken because (while unlikely) it might be stuck in their throats without proper chewing.