So you will hear it time and time again, worded differently depending on the situation... "You don't know what you've got til it's gone." And yes there is a song about it by Amy Grant, go take a listen.
It's about taking things and/or for granted. You take them for granted because they are there everyday to take care of you or be your friend or because it's something you own. But what about when they're gone. Or it's gone. That toy you hardly ever play with because it's always just there, or the stuff animal that you drag around everywhere without thought. What happens when those things are gone?
People in your life are a bit different and they can leave your life for a variety of reasons. Maybe you take them for granted and you grow apart. You can reach out and jump start that communication. What if they move and over time the distance becomes an issue. I stop sometimes and wonder about people in my past, if we'd have had the technology that is available today, would things be different? Worst case, they pass away and no matter how much you miss them you can't resolve that one.
So cherish the people and things in your life that you are fortunate enough to have and don't take them for granted, hold them close and take care of them.