The game is based on the Greens' emotional experiences of raising their son Joel, who was diagnosed with terminal cancer at twelve months old, and though only given a short time to live, continued to survive for four more years before eventually succumbing to the cancer in March 2014. The game is designed to have the player experience the low and high moments of this period, using the video game medium to provide more interactivity and immersion for the Greens' to relate their tale that a film cannot. Alongside the game, a documentary Thank You for Playing, documenting both the last few years of Joel's life and the development of the game, has been filmed to be aired in 2016.
Source: Wikipedia, "That Dragon, Cancer", available under the CC-BY-SA License.