An outside bar shows expansion: both sides were active and one of them finished on top. Where it closes matters most. A close near the high after taking out the prior low is a bullish reversal read; a close in the middle is just volatility.
These often coincide with stop runs. Price dips under the prior low, triggers sell stops, then reverses and closes above the prior high, which is a liquidity-sweep in candle form.
The practical difficulty is size. Outside bars are large by construction, so a stop beyond the bar creates significant risk, and entering mid-bar means guessing. Many traders wait for the next bar to hold above the outside bar's midpoint before committing, accepting a worse price for better odds.
Related: inside-bar, wide-range-bar, bullish-engulfing