Main axis

The main axis in flexbox is defined by the direction set by the flex-direction property. There are four possible values for flex-direction. These are:

Should you choose row or row-reverse then your main axis will run along the row in the inline direction.

!In this image the flex-direction is row which forms the main axis

Choose column or column-reverse and your main axis will run top to bottom of the page in the block direction.

!Three flex items taking up the full width of the container, displayed one below the other in code order. Flex-direction is set to column. The main axis is vertical i.e., from top to bottom