What does the flex-flow property do?

The flex-flow CSS shorthand property specifies the direction of a flex container, as well as its wrapping behaviour.

/* flex-flow: <'flex-direction'> */
flex-flow: row;
flex-flow: row-reverse;
flex-flow: column;
flex-flow: column-reverse;

/* flex-flow: <'flex-wrap'> */
flex-flow: nowrap;
flex-flow: wrap;
flex-flow: wrap-reverse;

/* flex-flow: <'flex-direction'> and <'flex-wrap'> */
flex-flow: row nowrap;
flex-flow: column wrap;
flex-flow: column-reverse wrap-reverse;

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

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