"Slab" is a relative term, that generally means "the bigger sized" fish of that particular body of water .... bigger than the average "keeper", and much bigger than a "dink" or "tater chip" (non-keeper or below minimum length fish).
Those look to be about 1-1.5lb Black Crappie ... excellent tablefare size, regardless of what "label/name" you give them!! ... luck2ya ...cp