Kevin DeYoung offers a great illustration of the reality of idolatry in his book, The Good News We Almost Forgot: Rediscovering the Gospel in a 16th Century Catechism:
Loving God is like loving your spouse: When you choose your mate, it is to the exclusion of all others. You can’t do both. You can’t tell your wife, “Honey, here’s my other lover. I really wanted you to meet. I know you’ll be great friends. You both mean so much to me.” Your wife will say, “It’s me or her. You take your pick.” No one would think this sort of wife cruel, or proud, or unfair, or intolerant for making such a demand. Monogamy is her right and her husband’s promise. The traditional marriage vows includes the phrase “forsaking all others” for a reason, because the nature of the relationship is one of exclusivity. The same is true in our relationship with God. He is jealous for our exclusive commitment.
May we love God with the same exclusivity that a man is called to love his wife and a wife her husband.
Idolatry is trusting in created things rather than the Creator for our hope and happiness, significance and security.