What is it when someone says the opposite of what he of she really means?

First, and most important, if there is any suspicion that a health issue is involved, you or the person involved must get checked out by a primary care physician. The answer depends on a few things. If done intentionally, it could be, I suppose, a blatant lie. If unintentional, it may be a simple 'Freudian Slip'. If this kind of thing happens and the person is flustered because she/he was really expecting 'what was meant' to come out and it didn't, and it happens often, there may possibly be some physical/medical impairment, or perhaps some issues of mental health.