What cryptic message does Julia hand to Winston on a small piece of paper? What does Winston think is the meaning of it?

At this point, he doesn't know her name is Julia. The message on the paper says, I love you. Winston is very confused about this. He is actually terrified of the girl thinking she is the thought police. Initially Winston thinks it is some type of trick by the thought police to implicate him. Later, he takes a chance by trying to meet her.