Why is there a number of lessons in grey in my preparations?

The curriculum that has been selected for the upcoming lessons, is displayed in your preparations. The number of these lessons is displayed in grey color:

Example as shown in the image above: 3 +1 / 4
- 2 number of lessons already taught about this topic
- 1 lesson about this topic planned for the upcoming future
- There are 3 lessons planned for this topic in your teaching plan

How can I plan the lesson topics for next lessons?