the.com/rap

means A rhythmic, rhyming style of vocal delivery performed over a beat — and also, more plainly, to speak or perform in that style.

from The music sense traces to African American vernacular 'rap' meaning 'to talk, especially fast and fluently,' prominent in 1960s street and radio slang ('rap session') before being applied to the emerging vocal style in 1970s hip-hop. That talk-sense itself likely descends from an older English 'rap' meaning a sharp blow or knock — the idea of words hitting hard and quick. The claim that it's an acronym for 'Rhythm And Poetry' is a popular backronym invented after the fact, not the real origin.