In practice
A bias narrows the day to one direction. If the higher timeframe is rising and price is heading for a clear target above, the trader looks only for long setups and ignores shorts, which removes half of the possible mistakes.
It comes from evidence that can be written down: the trend, where liquidity sits, and where price is within its range.
What to watch for
A bias is a working assumption. Holding on to it after the market has broken the structure it was based on turns it into stubbornness.