"Plans" is where you find access to the plans, and the groups, lessons, and study content under a plan.
A "Plan" is like a private course you can populate with courses, groups, lessons, study content, and individual study items. Plans are created or populated via "Add to plan" links found on many pages.
The "Content" page shows you the current lesson content item.
For the case of audio/visual media items, it will display a media player. For study list content items, it will list the study list items in a configurable way, and so forth.