Complex sentence: Statistical hypothesis tests define a procedure that controls (fixes) the probability of incorrectly "deciding" that a default position (null hypothesis) is incorrect based on how likely it would be for a set of observations to occur if the null hypothesis were true.
Split into simpler sentences: Statistical hypothesis tests define a procedure that controls (fixes) the probability of incorrectly "deciding" that a default position (null hypothesis) is incorrect. The procedure is based on how likely it would be for a set of observations to occur if the null hypothesis were true.