The align-content is similar to align-items, but instead of aligning flex items, it aligns lines. There must be multiple lines of items for this property to have any effect.

flex-start: Lines are packed toward the start of the flex container.

flex-end: Lines are packed toward the end of the flex container.

center: Lines are packed toward the center of the flex container.

stretch: Default value. Lines stretch to take up the remaining space.

space-between: Lines are evenly distributed in the flex container.

space-around: Lines are evenly distributed in the flex container, with half-size spaces on either end.

See the Pen Flexbox Align-Content by brandon ewoldt (@bewoldt) on CodePen.