A test of significance assesses the evidence provided by data against some claim (the null hypothesis H0). Data that are far from what we would expect if H0 were true are evidence that H0 is not true.
The basketball team at State University claims that each of their players makes 80% of his free throws. That's a null hypothesis. To test the claim for a player, ask him to shoot free throws and compare his results with the null hypothesis by clicking Show null hypothesis. A statistical test makes the comparison more precise by giving the probability of an outcome as far from the claim as the player's actual performance. When you are satisfied that the data have or have not falsified the null hypothesis, click Show true % to see the truth for this player. Click New Shooter to have a new player (i.e., different free throw average) try to prove his claim.