Coin slab is a type of holder for a coin. Slabbed coins are typically from one of the coin grading companies. The practice of sending coins to third-party grading companies and then "slabbing" them began in 1986.
When a grading company grades the coin it is sealed in a tamper proof slab with a barcode and a hologram. To prevent counterfeiting, holograms were attached to the graded coin slabs beginning in 1989. The early coin slabs did not have a hologram.