mustard
Explanation
The comic shows a man sitting on a couch with two women, making what appears to be a deeply emotional confession: "This... can you forgive me. I... I've never gotten over Roger and I never loved you. And I see each sexual encounter as a kind of penance -- atonement for the full love I've denied you as I long night after night for our -- oh God. I meant to say mustard."
The caption below reads: "It was the greatest Freudian slip of all time."
A Freudian slip (or "parapraxis") is a concept from Sigmund Freud's psychoanalytic theory, referring to an error in speech, memory, or action that supposedly reveals an unconscious desire, thought, or feeling. The classic example is accidentally calling your teacher "Mom," or saying one word when you mean another in a way that reveals hidden feelings.
The joke takes the concept of a Freudian slip to an absurd extreme. Normally, a Freudian slip is a single misplaced word -- saying "bed" instead of "bread," for example. Here, the man delivers an entire emotionally devastating confession about being in love with someone named Roger, never having loved his partner, and viewing their intimate life as penance -- and then claims this entire elaborate, deeply specific monologue was a slip when he simply meant to say "mustard." Presumably someone asked him to pass the mustard, or asked what condiment he wanted.
The humor lies in the sheer impossibility of the claim. A Freudian slip is typically one or two words; this man has delivered a full paragraph of gut-wrenching emotional truth and is trying to pass it off as accidentally saying the wrong thing. The caption's deadpan declaration that it was "the greatest Freudian slip of all time" plays along with the absurd premise, treating it as though it really were just a slip rather than an obvious (and spectacularly awkward) confession of his true feelings. The comic also touches on the common human tendency to try to walk back things we've said by claiming we "didn't mean it" -- taken here to its most extreme and least plausible form.