Here's an explanation of back crossing In more detail.

• Resistant means that they can survive the blight and can pass on this trait to their offspring.
• Susceptible means that if they are exposed to the blight, they will get infected and most likely not survive. This trait of being susceptible will get inherited by their offspring.
  
• Intermediate means that its resistance falls somewhere in between being completely susceptible or being completely resistant.
• A cross is the selection of both parents for breeding based on the desired traits of the parents.
• A back cross is the selection between an offspring of a cross and one of the parental types of that same cross.

First, a disease resistant Chinese chestnut is crossed to a susceptible American chestnut. The resulting seeds are planted and grown into small trees. Each small tree is tested to measure its resistance to blight. The most resistant tree is crossed to a susceptible American tree. This is called the first backcross.

As before, the resulting seeds are planted, grown into small trees, and tested for blight resistance. Once again, the most resistant tree is back crossed to an intermediate American tree. At each step in the process, the trees become more like the original American chestnuts, except that they are increasingly resistant to blight.