"Rizz" is short for charisma — specifically, the skill of attracting someone through talk alone.
Rizz is what you say and how you say it — confidence, timing, and not being afraid of a little risk in your words.
It means someone's vibe is so strong they don't even need words. Real rizz still usually needs the words.
Reading about it only gets you so far. Talk to someone new and see what happens.