A dictionary might say that a "friend" is "a person you know well and who you trust and like". A newspaper once offered a prize for the best definition of a friend.
One person said a friend is someone who "understands your silence." It's saying that friends do not feel embarrassed if they sit together without speaking to one another, or it means a friend will understand when you want to be alone or do not want to talk. Someone else said a friend was like a watch that "beats true for all time and never runs down." In other words, a ture friend's attitude towards you will not change over time.
The winner of the prize was the person who wrote, "A friend is one who comes in when the whole world has gone out." Other people may run away and leave you when you need help, but a true friend will stay by your side.