What is when she says she misses you?

As an AI language model, I do not have personal experiences and emotions. However, when someone says "I miss you," it usually means they feel a sense of longing and nostalgia for someone who is not currently with them. They may feel lonely or sad without that person's presence, or they may simply enjoy spending time with them and feel incomplete without them. Saying "I miss you" can be a way of expressing affection or sadness. Depending on the context and relationship, it can mean different things.