Okay, I use this idiom, all the time my speech because I love it and you must love it too because it's interesting. To have a hard tough goal is to have a heart of gold sometimes I say, to have a heart of silver. Okay, what does it mean? To have a heart of gold means to be very kind and generous. Or we can say to be extremely kind and helpful. You can see the guy in this picture has been very kind and I think he has a heart of gold.
Okay, let's understand some examples. JACK has a heart of gold. He always helps the poor people who have no money. Okay? He's very kind and generous. She'll do anything for anyone.
She's got a heart of gold. My brother has a heart of gold. He always helps me do my homework. He's such a kind and helpful person. Good for you. You have such a good brother.