it means sometimes after you haven't seen or spoken to someone in a while you may 'forget' what the person is actually like and start to remember them based on your own projections (i.e, what you 'wish' they were like or your fantasies of having them back etc). It's quite common for example in cases where someone starts to miss their ex or someone they used to be friends with despite the fact that the relationship had ended badly and the person hadn't been good to them. As time goes on the person may begin to forget or suppress all the negative emotions associated with their ex or old friend and suddenly start remembering positive things. So they're not actually missing the person, they're just missing a fantasy.
Another way of looking at it is like this. Lets say you went to a lake summer camp when you where 10 years old. You absolutely hated being there, the other kids bullied you, you didn't like the food and you were homesick the whole time. but you made 1 or 2 friends and did some things you enjoyed like swimming in the lake. Many years later, you're 17 years old and you suddenly start feeling nostalgic for the summer camp you went to as a kid. Enough time has passed that you don't fully remember or associate with the negative emotions and feelings you had when you were actually there, you only remember the good stuff (swimming in the lake, your one or two friends). Boy, you really do miss that summer camp. So here's the thing: you don't actually miss the summer camp (because though you dont fully remember it, it was actually a pretty crap experience) you're only missing the idea of it, that is, your nostalgic fantasy of what you think it had been like.
Hope that helps.