Knowing something they dont
They, in this context is the owner of the asset you are purchasing. To make super-normal returns this is fundamentally what you need to achieve. How do you consistently know something someone else doesn't without breaking the law? You can't. The best you can do is put the odds in your favour by looking for assets where you are competing against few people in the bidding process (small cap equities, unpopular industries, countries, businesses)
and a better understanding than them of the probability of possible outcomes.
A good analogy is poker. Even if you could find a way of knowing what card your opponent has it would be cheating and in the equities context therefore illegal. You can however increase your chances of winning by choosing your opponent and being better prepared. Competing against a crowd at the local Casino is likely to be afr easier than competing in national competitions for example. You can be better prepared by understanding statistics and Expected Values. For example you can get an idea of your chances of winning a hand simply with knowledge of what cards you have and what cards are on the table. You also have a good idea of what the prize is (the pot of winnings) and can therefore work out the maximum you should pay (bet) to play the hand. For example, if the pot is £100 and you think your chance of winning the hand is 75% you should be willing to pay up to £75 to play. There is still a chance you will lose the hand but over time, if your assessment of probabilities is corrrct, you should come out on top more often than not. The same is true of asset investing, you cannot know what will happen with any individual investment but if you have a good understanding of what drives pricing behaviour and are disciplined with investment sizing and risk assessment, over time you should make positive returns that should accumulate and compound over time.