What it means
Write down what would prove your bias wrong before the session, and look for it honestly.
In practice
Once a trader has a view, the mind starts collecting support for it. Bullish signals look significant, bearish ones get explained away, and the trade feels more certain than the evidence justifies.
The defence is to argue the other side on purpose. Before entering, write the case against the trade and name the price at which the idea is wrong. A stop at that price turns the answer into an order.
What to watch for
It is strongest after the trade is open, when money is at stake. Searching for opinions that agree with a losing position is the usual sign, and it is how a small loss gets held into a large one.