What it does
I'm not aware of "Affirmative" as an official Magic: The Gathering keyword or ability. It doesn't correspond to any keyword in the current comprehensive rules, and it's not a recognized mechanic from any set I know of.
It's possible you're thinking of a different term, a misremembered name, or something from an unofficial/custom card set. A few keywords that sound vaguely similar or might be what you mean include "Affinity," "Amass," or "Adapt"—or it could be flavor text rather than a rules term.
If you can tell me the card name, set, or where you saw "Affirmative," I'd be happy to clarify the actual ability involved. I'd rather not invent rules for something that doesn't exist, since that would only mislead you at the table.
Iconic cards with Affirmative
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