If you are well versed in the world of acting then you should have heard of Konstantin Stanislavski. Stanislavski was a Russian stage actor and director who came up with the Stanislavski method, which highlights a number of techniques an actor can use to help them identify with the character they are portraying.
The Magic If is based on a series of questions actors should ask themselves centred around the word if, to help them gain a better understanding of their characters future actions and emotions. For example in any scene where that actors character needs to respond to the situation they should ask themselves "How would I respond if I was asked that question?", "What would I do if I were facing these problems?", "How would I feel if I was going through this?".
The whole concept of "The Magic If" is to understand the character you are trying to portray from a psychological stand point, and use that understanding to display convincing and seemingly real emotions and actions by the actor.
By placing themselves in the characters situation, they are gaining an understanding of how they themselves would personally react IF they were to have gone through the same situation, which in turn would lead to the actor taking truthful actions when performing the role.
Not to be confused with method acting, a popular technique known amongst many and developed on the acting model Stanislavski put in place in the 1900s. The 'Magic If' focuses more on the actor drawing from their own personal experiences and understanding of themselves and their behaviour to then apply that to the character they wish to portray. Where as method acting is a primarily action-based method that requires the actor to fully inhibit the role of their character.