This lecture introduces the idea of a formal logical fallacy—that is, an argument that does not follow from its premises. Affirming the consequent is one such fallacy.
Recall that modus ponens states that the premises
P => Q
P
allow us to conclude Q.
Affirming the consequent attempts to make a similar argument using the premises
P => Q
Q
to conclude P. But these two premises do not allow us to conclude anything about Q; Q may or may not be true.
We call it “affirming the consequent” because Q is the consequent in the implication and the second premise affirms it.