The author of Into the Wild states in his introduction that, "in trying to understand McCandless, I inevitably came to reflect on other, larger subjects as well: the grip Wilderness has on the American imagination, the allure high risk activities hold for young men of a certain mind, the complicated, highly charged bond that exists between fathers and sons."
I believe the author is implying that the book addresses these themes.
One significant theme in Into the Wild is the deep and secret alienation that Chris felt for his parents. He was very angry at him, although his complaints are clear. The reader knows he's angry, but isn't sure why. We see from this relationship the different standard of morals that Chris had for his family and his friends. He was much more lenient towards his friends, forgiving them for things which he would never forgive his father for.
Another theme was Chris' desire for exploration and explanation. Chris wanted to travel because he wanted to find out about mankind. However, so stubborn was Chris that he did things his way, and didn't listen to the preparations he was told to undertake. Chris had a problem taking advice, either from the experts or his friends/family.
The Lochness Monster
A big symbol in Jon Krakeur's Into the Wild, is in the beginning when Chris McCandless burns his money:
"He changed his name, gave the entire balance of a twenty-four-thousand-dollar savings account to charity, abandoned his car and most of his possessions, burned all the cash in his wallet."
This shows how he did not need material posssessions while someone else who fit that need, would need them. Being truly free means letting go of all possessions and leaving the past behind. It shows how the character wants a fresh start in life and to be self sufficient in the wild and create his own 'world'. He wants to become disconnnected from the rest of society.