Monty Python actually has a sketch poking fun at how impossible it is "summarize" Proust, but I'd say his books are about imagination above all, and the role it plays in art, love, and our social lives. In very broad thematic way, one could say it's about the relationship between truth and illusion. There is a shortish book called "Days of Reading" that contains essays by Proust that make a digestible introduction to the style and thoughts of "In Search of Lost Time."
It's hard to nail down the meandering plot, but it focuses primarily on a sensitive and sickly protagonist, not unlike Proust himself, and his infatuations and movements through "high society." This is interspersed with ruminations on art and love and time and whatever. Proust had an amateur interest in philosophy, like Kant and Schopenhauer, so he engages with some pretty heady stuff at times.