Watch the Open and Closed online guitar lesson by Matthieu Brandt from Songwriting on Guitar
Open and closed refers to the amount of energy you feel at the end of a section. We call it open when the progression wants to move forward and it feels like there is more to come. If you don't feel any momentum anymore and the section feels at rest, we call it closed. Each element within the song has an influence on the amount of closedness and openness you feel: harmony, melody, rhythm and lyrics.
Toying with all these elements within your composition is what makes writing fun and sometimes a bit of a puzzle.