If you take something for granted, that means you think it will always be there for you, or it will always be given to you. "To grant (someone) (something)" is much like "to give", except the person or group who "grants" is a much higher status, eg. parents, a judge, a government, an employer and so on.

If you take something for granted, it means you don't respect or recognise how valuable that thing is. When it's gone, you'll really miss it!

"Buying a house is a great investment, because the prices of houses only go up." "Don't take that for granted."

"We could never have a tyrannical dictator in our country, because of our laws and traditions." "Don't take that for granted."

"You never thank me for the work I do around the house. Don't take me for granted."

If explained in simple words, "don't take it for granted" means do not think you can obtain something without paying, or do not believe your ideas/conclusions are correct without analyzing. Samples can be:

1. You need to work, if you want to get paid. "Don't take it for granted that money grows on the tree". 

2. You need to spend some time learning English, if you want to speak it fluently. "Don't take it for granted that you can speak English as well as your mother tongue without learning hard".

3. Communication is a trained skill, not something born with you. "Don't take it for granted that you can give a good speech to the public without training".