What does line $19$ in the code do$?$

It renames each of the objects as just "Clouds", so there are still three objects but all with the same name.

It does nothing. You could comment out line $19$ and nothing would change about the code.

It creates a list called "Clouds" and adds each object to the list. There are still three objects, but they are now grouped together.

It merges the three cloud objects into a single cloud, so there is only one big object now.