A posting plan in Post Planner — also called a "plan" — is a daily schedule that determines when your planned posts go out or publish.
For example, if you want to post to your Facebook page 3 times a day on weekdays, you can create a posting plan in Post Planner to make that happen.
To do so, you go to PLANS, choose your 3 times to publish each weekday, choose which kinds of content you want to publish at those times, and voilà... you're set!
Then all you need to do going forward is to create the posts and "plan" them — ie. queue them up to go out according to your posting plan.
You do this by clicking the "Add to Plan" button in the post composer.
Here's a video overview of posting plans in Post Planner:
In the video, you learn
The difference between the Main plan and Custom plans
How to create a Custom plan
How to configure your plans: time zone, day groups, time slots, content
How to see your plans manifested in the POSTS section