Do you remember what adverbs are? You would only need 'the' in this sentence if you were asking someone if they knew the specific adverb in one instance - do you know what the adverb is? (in this sentence). "The' in that case points to one specific thing. A general enquiry as to whether someone knows what adverbs (in general) are would be said as I wrote at the beginning.

The second is correct: "Do you remember what the 'Adverbs' are?"

Unless the Adverbs are the name of a band, it should be "Do you remember what adverbs are?"