Gold is a bright yellow metal. It is a very precious and useful thing. There are things that glitter like gold. But they are not precious like gold. They do not possess the virtues of gold. So the proverb means in a general way that we should not judge a thing merely by its outward appearance. The bright outward appearance often deceives us. We may be charmed by the outward appearance of a man. He is finely dressed and looks like a perfect gentleman.
Image Source: upload.wikimedia.org/wikipedia/commons/1/15/Gold-gold_venezuela_meieran1.jpg
But in reality he may be a cheat. Thus vice often puts on the garb of virtue. A hypocrite may seem to be a sincere person. The outward shows often delude the youth. Their judgment has not been matured. They are therefore easily misled by the deceptive appearances. We make friends with persons who seem to be sincere friends, bust they may turn out to be false friends and they may cheat or ruin us. A showy thing often proves to be worthless thing. So we should judge the quality of a thing carefully. We should remember that things are not always what they seem.