Focus on the smallest possible constituent of an utterance. Prime examples of minimal focus are utterances where the polarity of the verb is regarded as the only new information. See the two converstional exchanges below, where underlining indicates the place of the intonational nucleus:

A: He said he’s sent it
B: Well, he hasn’t sent it.

Here the nucleus placement indicates minimal focus on the negative polarity of the verb.

A: I thought he liked classical music.
B: He does like classical music.

Here the positive polarity of the verb receives minimal focus.