We all have things in our lives that are important to us. Often they are good things, like family or getting good grades at school. But if you focus on them too much, no matter what it is, it can become an idol.
An idol is anything that has become more important to us than God. If we focus on school more than God, it might become an idol. If we make our family the focal point of our lives, they can become an idol. If the thing we think about all the time is work, than it might be an idol.
Not all idols are bad things. For the most part, the things listed above are all good. Depending on the situation focusing on them more than God might not be an issue. If you have a sick kid, it make sense to focus more on them. Finals are coming up and you chose to study instead of spend time with God. That is understandable. Anyone who works in retail can tell you stories about having to skip church because it is two days before Christmas and a new shipment of the hottest toy of the year came in. It happens. It is when this becomes the norm, not the exception, that this is a problem
Keep you focus on God. Make Him the center of your life. Ask Him to show you things you have made more important than Him. Set your mind on God, because nothing is more important than Him.