'Qualia' is a philosophers' term of art denoting those intrinsic or monadic properties of our sensations discriminated in introspection. The quale of a sensation is typically contrasted with its causal, relational, or functional features, and herein lies a problem for functionalism. The quale of a given sensation - pain, say - is at best contingently connected with the causal or functional properties of that state; and yet common intuitions insist that said quale is an essential element of pain, on some views, the essential element. Functionalism, it is concluded, provides an inadequate account of our mental states.
Before addressing the issues in greater detail, let us be clear about what the functionalist need not deny. He need not and should not deny that our sensations have intrinsic properties, and he should agree as well that those properties are the principal means of our introspective discrimination of one kind of sensation from another. What he is committed to denying is that any particular essential to the identity of any particular type of mental state. Initially they may seem to be essential, but reflection will reveal that they do not have and should not be conceded that status.
References
CHURCHLAND, PAUL M., and PATRICIA S. CHURCHLAND. 1981. “Functionalism, Qualia, and Intentionality.” Philosophical Topics 12 (1): 121–45.