A vertical line in an array; A vertical arrangement of objects.
A rectangular array of objects consists of rows and columns of those objects. This rectangular array of squares can be described as consisting of two rows of four squares each, or as four columns of two squares each, or as having two rows and four columns of squares.
Columns run vertically, like the columns of a building. In these pictures the first and third columns are indicated by a red loop.
Notice that in the following two pictures, the same collection of dots is looped.
That collection of dots is the second row in one picture and the second column in the other.
If an array is held so that there is no obvious vertical or horizontal, the words "row" and "column" are generally not used, because they become ambiguous.