RTP (Return to Player) is the average percentage of money a casino game pays back to players over time. If a game has 96% RTP, it means that for every $100 wagered, it's designed to pay back $96 on average, with the casino keeping $4. That doesn't mean any one session pays out exactly that — it's a long-run average across thousands or millions of plays, and short-term results (variance) can swing well above or below it.
Every game sets its own RTP. Each slot, table game, or other title has its own fixed RTP built into its math and paytable, so the percentage can (and does) vary from one game to the next.