React: What is React key warning?

Warning: "Each child in a list should have a unique key prop"

Causes: Missing key prop, duplicate keys, using index as key.

Fix:

1// Missing key
2{items.map(item => <Item {...item} />)}
3// Unique key
4{items.map(item => <Item key={item.id} {...item} />)}

Impact of ignoring: Incorrect DOM updates, state preservation bugs, performance issues, animation glitches.

Debugging: Check data for unique IDs, verify no duplicate keys, use React DevTools.