Simply put, hypnosis is a state of hyper-focused and relaxed attention where you are more open to suggestions. 

It's similar to when you are so absorbed in a good book or movie that you lose track of time and your subconscious actually responds to the plot as if it was really happening.

In a similar way, the hypnotherapist helps you enter a hypnotic state through progressive relaxation, and your subconscious will likely accept the suggestions provided as fact and then internalize them as its own beliefs.