Lesson in Titling Your Book: When in doubt, name it after your protagonist. Hamlet, Jane Eyre, and David Copperfield are just a few examples of some great writers taking the easy way out of titling their works. Zelazny would have followed suit, only you can't name a science fiction epic Sam—that's just silly. So, Lord of Light it is since Sam is the Lord of Light.
It is that simple, right? Maybe not… Sam goes by many names in the novel, including "Mahasamatman, Kalkin, Manjusri, Siddhartha, Tathagatha, Sinder, Maitreya, the Englightened One, [and] Buddha" (1.93). Any one one of these names could have been chosen for the title and sounded way less vanilla than Sam. So why go with Lord of Light?
There are many possibilities, really, as this book is big on allowing for many answers rather than having an answer. For our money, it's a matter of the journey Sam and the world undergo during the story.
Sam starts the novel off as a self-centered prince who only takes on the image of Buddha to counter the gods. In the last chapter, Sam claims people only call him the Buddha because they "are afflicted with language and ignorance," but Taraka denies this, believing Sam is "what [he] claimed to be" (7.278-279). Lies or no, Sam has transformed into an Enlightened One, a Buddha, a true Lord of Light.
At the same time, Sam brings a sense of Enlightenment to the world. Much like our own age of Enlightenment in the 17th and 18th centuries, Sam's world learns to be less reliant on the gods and more reliant on reasoning, invention, and scientific observation. It's important to note that the humans were re-inventing certain technologies before Heaven's doors were opened wide for them to access the Celestial City's scientific goodies.
The novel is named Lord of Light to signify this play on the religious and philosophical versions of the word enlightenment. Sam is the Lord of Light because he brings himself and the world out of a philosophical and religious dark age.