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You've seen what a single neuron does: it takes in inputs, multiplies each one by a weight, adds them up, and fires if the total crosses a threshold. That's the whole operation.
Think about what that operation actually does. It adds everything up and asks: is the total above the threshold? If yes, fire. If no, don't. That's one question, asked once. Everything on one side of that question is "yes." Everything on the other side is "no."
That's what people mean when they say a neuron can only draw one line. Not a line you can see, a single dividing question that splits the world in two.
For simple problems, that's enough. Is this number bigger than five? Is this email short or long? One rule can answer those.
But most real problems aren't like that. Think about sorting photographs into cats and dogs. The difference between a cat and a dog isn't one thing you can measure and compare. It's a combination of things: the shape of the ears, the structure of the face, the proportions of the body. None of those features, on its own, gives you the answer. The answer lives in how they combine.
One neuron, one rule, one line. It can't find that.