It's a special sort of feeling that is very close to the mental state known as happiness. The difference is that it has a physical presence.
There are different things that can trigger it: receiving a unexpected and kind act, meeting with someone you care for, experiencing a beautiful sight or actual warmth from the sun or other heat source.
I always remember when I would be at school early, sitting at a cafeteria table by myself. I would look up to see the familiar face of a friend coming over to join me. I recall that I couldn't help but form a stupid grin naturally as I would feel warmth in my chest. 'This person is here for me, as my friend, because they care and I matter to them'. Everyone deserves to have this, especially children.
I can't imagine a life without warmth, to live in a world of cold. We should all share our warmth, give our friends and family stupid grins and make their heart skip a beat. It's the greatest and most sincere gift.