Determinism is the claim that every event, including every choice you make, is the necessary consequence of prior conditions plus the laws of nature. If you rewound the universe to last Tuesday with everything identical, the same Wednesday would unfold. There's no genuine alternative; the future is fixed.
The thesis is surprisingly hard to dismiss. Most of what we know about the physical world looks deterministic. Even quantum mechanics, which introduces randomness, doesn't obviously help — random isn't the same as free. And the evidence from neuroscience increasingly suggests that the brain commits to a decision milliseconds before consciousness catches up.
What's at stake is moral responsibility. If you couldn't have done otherwise, can you really be blamed? Compatibilists (the majority position in academic philosophy) say yes — "could have done otherwise" should be understood as "would have done otherwise if you'd wanted to." That's enough for praise, blame, and the practices that hold society together. Hard determinists say no, the whole apparatus of guilt and merit is built on an illusion, and we should reorient our institutions accordingly — focusing on rehabilitation rather than punishment. The debate has real-world consequences for criminal justice, addiction policy, and how we treat people who fail.