Strong trend? Lots of factors in your favour? Technical + fundamental confluence? Divergence? Strong levels?
It's a term I associate mostly with win-rates, really - so in itself it has only limited relevance to me, as part of a bigger probabilistic/statistical picture (the "highest win-rate set-ups" are what most people mean by "highest probability set-ups", and they're not often going to be the ones I'd want to trade, myself). That said, and to answer your question, what makes a set-up "high probability" to me is always current, valid, statistical evidence that its win-rate is higher than whatever it's being compared with (which may or may not also make it more profitable). For myself, I more or less ignore fundamentals unless I've been specifically notified of their likely relevance to me (that's easy for me to say as a fast-moving intraday trader, of course!), so it would be "technicals", really. Certainly including strong levels of support and resistance, when there are some. And I generally consider the most frequently touched/rejected and the most recently touched/rejected levels of previous support and resistance to have higher-than-average chances of becoming short-term future levels of support and resistance. For what it's worth. In view of the way you've asked your question, I suspect that Marcel Link's book High Probability Trading might interest you. It's not bad at all, as these things go. His principle contention is that similarity of patterns (whether price action or indicator-based) across multiple time-frames makes for "high probability set-ups". I don't use time-frames, myself, but I used to and I know what he means and he's right. But - again - don't make the mistake of imagining that the highest win-rate trades are necessarily also going to be the most profitable or indeed the best (judged by any other parameters) to trade. Would that it were that simple!
You should never assume a certain trade will be "HIGH PROBABILITY" ...assuming... is the mother of all fucks ups. Constantly try to keep an open, neutral, calm mind...while still having a certain degree of directional conviction, and supporting loose logic and rationale behind that stance. After you've been trading the same thing for a while...you'll kind of develop a sixth sense of when 'high probability' instances are about to happen and unfold.
When I can be pretty confident the buying or selling I'm seeing in the market is fresh opening of positions by the bigger players.
I believe my edge increases when most long/short traders have puked out in very high volatility/volume events. Those that preserved capital and have no recent pain/fear tied to the move can clean up on others mistakes. The impatient crowd jumps in to soon and gets clobbered in violent whipsaws. The more skilled traders are looking for a few rinse cycles to fakeout/shakeout the last of the weak hands prior to a big directional move. Reward/risk can hit extremes, these are the conditions ripe for a 500-10,000+ basis points for those that hold positions, skilled day traders are going to do well to of course. These extremes market conditions are the reason I trade period. I don't believe in high probability in trading - I see it as skewed reward/risk & opportunities.
I like this viewpoint. For me, every trade I take is high probability, so I set entry orders to the fullest extent my risk management will allow, as soon as an entry signal prints. I put in pyramid orders on every trade opened, in anticipation that at least some of them, though I have no knowledge of which, will run on in my direction.
Extreme emotions equates high probability. There is a direct correlation. That's why novices have to wait patiently for these moments, algos lay in wait for these events.
Watching this video can be insightful. Consider the length of the pendulum wire is the trading frequency of a particular strategy/set-up. There’s trending and there is overlap of trends (which can be consider sideways or chop). Each strategy is independent from the others yet at times coherent to the overall perception of directional movement. The concepts one uses of FA/TA build the structure by which one perceives market movement.
If we continue the analogy, longer pendulum wire lenegths are larger accounts that trade longer intervals and smooth drawdowns. A smaller account (smaller wire) attempts to out perform the larger by trading through the larger oscillation to not experience the drawdown. This relationship between the two continues in both directions of the spectrum. Using this model, one sees many patterns arise from all the different strategies involved in market action. One could also see the patterns themselves are something we impose on the market. The market cycles. What are the necessary components of the market to understand this cycling?