A wandering mind is a common occurrence. Minds wander on average 30% of the time and can wander as much as 70% when driving.
A wandering mind can be a good thing because it's the closest thing to multi-tasking that humans can achieve. For example, you might be doing your homework but thinking about what to bring to a party you've been invited to attend the following evening.
A wandering mind can be a bad thing because if you zone out, you might miss important pieces of needed information causing you to be unproductive.

29. Minds Wander 30 Percent of the Time