Ben,

"Slabbing" is the process of submitting an item (in this case a chip) to a third party authentification/grading service. The service supposedly expertly and unbiasedly grades the chip. The graded chip is then placed in a case ("slab") which is supposedly tamper proof. The idea is that by doing this a person can rely on the grade assigned by the slabber.

One major problem with this is that casino chips have no recognized grading system.

In coin collecting miniscule differences in the numerical grade of a chip can amount to large price differentials. This leads to abuses in the prcatice of slabbing.