In awk,
Since 1 always evaluates to true, it performs default operation {print $0}, hence prints the current line stored in $0
So, awk '$2=="no"{$3="N/A"}1' file is equivalent to and shorthand of
awk '$2=="no"{$3="N/A"} {print $0}' file
Again $0 is default argument to print, so you could also write
awk '$2=="no"{$3="N/A"} {print}' file
In-fact you could also use any non-zero number or any condition which always evaluates to true in place of 1