A raw card is one that hasn't been graded and slabbed by a third-party service like PSA, BGS, or SGC. When you rip open a pack of Topps cards, every card you pull is raw — ungraded, unencased, and in its natural state.
Raw cards are generally less expensive than their graded equivalents in the same condition, because there's no third-party verification of quality. That said, buying raw can be a smart play if you have a good eye for condition — you can scoop up cards at a discount, submit them for grading yourself, and potentially unlock significant value if they come back at high grades.
The raw-vs-graded debate is eternal in the hobby, but both have their place in every collection.