What does the flex-direction property do?

The flex-direction CSS property sets how flex items are placed in the flex container defining the main axis and the direction (normal or reversed).

/* The direction text is laid out in a line */
flex-direction: row;

/* Like <row>, but reversed */
flex-direction: row-reverse;

/* The direction in which lines of text are stacked */
flex-direction: column;

/* Like <column>, but reversed */
flex-direction: column-reverse;

/* Global values */
flex-direction: inherit;
flex-direction: initial;
flex-direction: unset;

Source: https://developer.mozilla.org/en-US/docs/Web/CSS/flex-direction