To "take something for granted" means that you are not appreciating something the way you should be. That term is often used in describing something you no longer have that you didn't think was so important when you had it. For example, if someone always makes you dinner you might sometimes complain about what they chose to make. Then one day they stop making you dinner and you have to make it yourself. You might miss them making dinner for you even though you complained about the choices. In that situation you took them making dinner for granted. Sometimes older people use that phrase as a warning to younger people "Don't take that for granted, you'll miss it when it's gone."
Hope this helps. Keep learning :-)