For me it means: appreciate what you have today because tomorrow it/she/he could be gone, so say I love you today, send that card, make a call, tomorrow it could be too late. Ok, a less mushy answer, "things" represent your world and what's in it, "for granted" means 'without thought', ergo, when you take things for granted you're absent, ignoring, not looking at the persons/things right in front of you. It's not a compliment.
Don't take things as it is, sometimes there's more to it beneath the surface of the words or it isn't true or it can be changed or use another way. Even take things for granted isn't simple to explain.
Be appreciative of what you have because you don't know if you will have it forever. "Granted" means handed over as an entitlement that will always be there, so it's saying don't expect that. Be appreciative and truly thankful for everything you have and savor everything like it's the last time.
It can mean try and think what planning, work and effort has gone into making something you enjoy available. Then be grateful.
It also means value something that may not be here forever, like its hard to imagine how we will feel when folks die, so value them every day, lots of hugs, and never have such a fight that you have "burnt your bridges" back to them. Sooner or later a time will come when they seem to be the only people on your side. When they have gone you may wish you could cook as good as they did, so learn now, they'll love the attention too.