The characters in The God of Small Things are constantly coming up against the forces of society and class. Indian society was structured for centuries according to very rigid social classes and boundaries, through what is known as the caste system. Even though the novel takes place after the caste system stopped being a legal social policy, its characters still find themselves limited by what is and isn't deemed socially acceptable for them. Social rules dictate who can love whom, which occupations people can adopt, and who is considered to be better than whom. (Sounds a little like an extreme version of high school, doesn't it?)
The characters of The God of Small Things are ultimately constrained and held back by class boundaries.
In the end, the characters of The God of Small Things show us that class boundaries are breakable.