How much return a strategy earns for the losses it actually makes you feel.
The Sortino ratio measures how much return you get for the downsides. It only counts the moves that lose you money, because nobody's ever been upset about a sharp move in their favour, right? Higher Sortino means more reward for the pain you actually feel. Most healthy systems land between 1 and 2 on a per trade basis.
It divides your return by your downside deviation. Sharpe uses total volatility, so it treats a big winning week as risk... which is a strange thing to punish. For anything with a fat right tail, a trend system that bleeds small and pops big, Sortino gives the fairer read than other metrics.
Use Sortino when your returns are asymmetrical, which most real strategies are. Use Sharpe when you want to talk to the wider industry, since it's the number everyone already knows. A momentum system can post a mediocre Sharpe and a strong Sortino at once, and the Sortino is the honest one. Neither means much on a tiny sample, so don't read either off 25 trades and call it done.
Per trade, above 1 is solid and above 2 is strong. Be suspicious of anything much higher on a short record. Annualised figures look bigger and follow different rules, so a Sortino of 2 per trade isn't the same as a Sortino of 2 for the year. Check which one you're looking at before comparing two systems, or you'll be lining up two numbers that were never the same thing, and this is important.