Postponed or On Hold are special categories, to which Plans and To-do's not finished during a week get automatically moved at report deadline.
Example: Your report deadline is Friday 5 PM. You have 5 items under your Plans and you mark three of them Done. Two items stay under Plans. When it's 5 PM on Friday, those two items under Plans get automatically moved to Postponed or On Hold, so your Plans section is clean for a fresh week.
You can use that category as an automatic or manual holding area for ideas and things you will work on in the future, but not this week.
As an administrator, you can rename the category to your own liking under Template Settings.
You can disable this automatic category and leave all planned items under Plans by checking the "Carry items over to next week" checkbox under template Settings. Again, you can do that only as an administrator.