What it means
After a strong move, a doji can be an early sign the move is losing steam. On its own it predicts little.
In practice
A doji shows a period in which neither side gained ground. Traders take note when one prints after a long run into an important level, because it is the first hint that the side in control is no longer pushing as hard.
The candle that follows matters more than the doji itself. A strong close against the earlier move gives the hint some weight; a close in the old direction means the pause was only a pause.
What to watch for
Dojis appear constantly in quiet, sideways markets and on low timeframes, where they mean nothing at all. Trading every one as a reversal signal is a quick way to lose to noise.