Very simply, meditation is an attempt at single-pointed concentration. It involves cultivating awareness of the mind's contents and the ability to let thoughts pass without fixation. "Zoning out in the shower" probably means something more like daydreaming, where any and all thoughts are permitted to exist without active control. Focusing intently on a difficult cognitive task ("flow state") is more akin to meditation than zoning out.

A lot of beginners are so bad at this that some amount of guiding back to the goal is helpful. Many can only go a few seconds without getting fixated on passing thoughts.

Practicing one's ability to focus on a single thing and reducing mind-wandering will improve one's capability for concentration.