What does the lighthouse symbolize in To the Lighthouse?

As something both illuminating and out of reach, the lighthouse symbolizes different things for different characters. The existence of multiple standpoints is a common feature of literary modernism, especially in the works of Virginia Woolf. Therefore, the lighthouse does not symbolize just one thing; it is open to a wide variety of interpretations.
For little James, it's a source of childish excitement. He constantly badgers his mother to take him on the short trip across the bay to this beguiling source of light. Yet for one reason or another, the trip never takes place. The lighthouse, then, could be said to represent the sense of wonder each of us has in our childhood, but which we subsequently lose once we reach adulthood.
As for Mr. Ramsay, the lighthouse symbolizes what he fondly believes is the love he shares with his wife. Like the lighthouse, this is supposed to be something permanent and illuminating. Yet like little James, Mr. Ramsay's perspective is partial and distorted. He doesn't realize that his feelings for his wife are not fully reciprocated. It's instructive that Mrs. Ramsay can't even bring herself to say that she loves her husband. That's not to say that she doesn't love him; it's just that her feelings for him are not as intense as his are for her.