The titular lighthouse of Virginia Woolf's masterpiece of stream-of-consciousness prose To the Lighthouse is both a real place and a symbolic one.
In Part One, Mrs. Ramsey is knitting a present for the child of the lighthouse keeper. Her young son is very excited about going to the lighthouse in their boat the next day to deliver the present, but her husband is adamant that the weather will not allow it. Mrs. Ramsey, for her part, would rather let the boy think the journey is a possibility than crush his hopes with practical concerns. Throughout this entire section, Mr. Ramsey is more concerned with his internal intellectual progress than with the details of the mild domestic drama unfolding around him. He worries he has reached an intellectual plateau and is desperately trying to get beyond the limits of his own mind. Thus we can see that the lighthouse means different things for each character in Part One: for Mrs Ramsey, with her concern for both her own child and the child of the lighthouse keeper, the lighthouse represents family; for her son James, who so adamantly wants to go, it could perhaps be understood to represent adventure; and for Mr. Ramsey the idea of going to the lighthouse is entirely metaphorical—the lighthouse is a symbol of brilliance and thought, a beacon towards which he is struggling.
It is not until Part Three that any of the characters actually go to the lighthouse. Ten years have passed, and Mrs. Ramsey is deceased. In this section, Mr. Ramsey takes two of his (now grown) children out on a boat to the lighthouse. His children do not remember their childhood fascination with going to the lighthouse and don't understand why the excursion seems to mean so much to their father. For his part, Mr. Ramsey is regretful that he hadn't been more supportive of the notion while his wife was still alive.
In Part Three, which is built around Mrs. Ramsey's absence and delves into how dearly each separate character misses her, the lighthouse becomes primarily a symbol for Mrs. Ramsey herself. What does a lighthouse do? It guides ships through the dark and fog safely home to harbor. Mrs. Ramsey was a master of smoothing social interactions to keep all involved happy, guiding conversation from risky waters to safety. As the matriarch of her family, she was fiercely protective of her children and her husband. Without her, the people she has left behind feel themselves to be adrift in the dark.