The title refers to the difficult journey of life and how it's possible to navigate through it successfully, even during the hard times. It refers to how a person has to be able to control themselves and their reactions even when times are hard. Garth Stein writes,
I know this much about racing in the rain. I know it is about balance. It is about anticipation and patience. I know all of the driving skills that are necessary for one to be successful in the rain. But racing in the rain is also about the mind! It is about owning one’s own body. About believing that one’s car is merely an extension of one’s body. About believing that the track is an extension of the car, and the rain is an extension of the track, and the sky is an extension of the rain. It is about believing that you are not you; you are everything. And everything is you.
Enzo's owner Denny is a race car driver. One thing he's very good at is racing the car even when it's raining outside and conditions are more perilous than normal. It takes a lot of skill, training, and bravery to be able to race in the rain successfully.
Life is like a race. Sometimes conditions are dangerous or downright bad. This happens to Denny several times in the book, like when his wife dies, when he has to fight for custody, or when Annika accuses him of rape. Staying on track during these times is hard, and coming out successfully is even harder. But Denny has to learn to "race in the rain" in order to be successful.