A slabbed card is a graded card that's been sealed inside a tamper-proof plastic case — the "slab" — by a professional grading company like PSA, BGS, SGC, or CGC. The slab protects the card and prominently displays the assigned grade.
Once a card is slabbed, it stays in that case. Cracking it open defeats the entire purpose, because you'd lose the authenticated grade. The slab is both a protective shell and a trust signal — buyers know exactly what condition they're getting without having to inspect the card themselves.
Slabbed cards generally command higher prices than raw cards in equivalent condition, because the guesswork is removed. If you're selling, a high-grade slab can significantly increase what you get. If you're buying, a slab gives you confidence in your purchase.