Expectancy in plain words, and why it beats every other metric

Take your average win, multiply by your win rate. Take your average loss, multiply by your loss rate. Subtract the second from the first. That's what you make per trade on average. If it's positive, you have something worth sizing. If it's negative, no amount of position sizing or psychology fixes it, sizing changes how fast you get to the outcome, not what the outcome is. This is why I get impatient with "improve your discipline" advice given to someone whose expectancy is negative. Discipline applied to a losing system just makes you lose more consistently.