Use the key prop when rendering lists to improve performance. When rendering a list of items in React, it is important to provide a unique identifier for each item. This allows React to efficiently update the list when changes are made, without having to re-render the entire list. The key prop is used to provide this unique identifier.

The key prop should be set to a value that is unique for each item in the list. This can be an ID or any other unique identifier. It is important to note that the key prop should not be set to the index of the item in the list, as this can lead to performance issues when items are added or removed from the list.

Here is an example of how to use the key prop when rendering a list of items in React:

{`function ItemList({ items }) {
  return (
    <ul>
      {items.map(item => (
        <li key={item.id}>{item.name}</li>
      ))}
    </ul>
  );
}

const items = [
  { id: 1, name: 'Item 1' },
  { id: 2, name: 'Item 2' },
  { id: 3, name: 'Item 3' },
];

ReactDOM.render(<ItemList items={items} />, document.getElementById('root'));`}

In the example above, the key prop is set to the "id" property of each item in the list. This ensures that each item has a unique identifier, which allows React to efficiently update the list when changes are made.