What did this lesson mean by this quote?

See screenshot :
The quote:
Black's 2 flanks are connected by only 2 lines(7th and 8th rank), and, therefore, they cannot be left in the centre.

Month 3: Piece Out of Play wrote:

1. What did they mean by the 2 flanks being connected by only 2 lines? How?
2. When they say "they" (they cannot be left in the centre), what "they" does the author refer to? What can't be left in the centre?

Cheers!

Black is at a space disadvantage in a closed center formation. Only the 7th and 8th ranks are available to move pieces between the Queen and Kingside.

In the game White is able to defend the Queenside and the a5 knight remains out of play as White builds a local superiority on the kingside. The reason is only the d7 and e7 squares connect the two sides of the board for black. Place the knights there and the bishops are out of play. Damned if you do, Damned if you don't; it's a difficult game when you are a piece down...

Oh, with "they", the author meant the [obstructing] pieces. Thanks!