ANSWER: Idolatry is trusting in created things rather than the Creator for our hope and happiness, significance and security.
When we studied the eighth catechism question we looked at two passages: Exodus 24:3-8 and 32:1-6. Those passages are worth revisiting now that we're looking at the problem of idolatry. Can you describe some of the ways that the Israelites' trust shifted from the Creator to the golden calf? We don't know every thought that went through their minds, but can you get some idea from these verses of why they switched from God to the idol? What was their reasoning?
According to Paul in Romans 1:18-32, what happens to us when we make a habit of serving idols? (This link will take you to both the ESV and Message translations--both are helpful for answering the question!)
Can an iPhone become an idol or, at least, a means to idolatry? What do you think of this analysis of screen-swiping as liturgical practice:
A way of relating to a phone [that is, swiping a screen to get what you want or get away from what you don't want] has now become a way of relating to the world. The practices for manipulating a small device are now expanded to show how we'd really like to manipulate our environment to serve our needs and be subject to our whims. And while we don't go around swiping our hands in front of us to change the scenery, we perhaps nonetheless unconsciously begin to expect the world to conform to our wishes as our iPhone does. Or I implicitly begin to expect that I am the center of my own environments, and that what surrounds me exists for me. In short, my relation to my iPhone--which seems insignificant--is writ large as an iPhone-ized relation to the world, an iPhone-ization of my world(view).--James K.A. Smith, Imagining the Kingdom: How Worship Works (Baker, 2013), p. 143.