The 2 axes are main axis and the cross axis. The main axis is defined by the flex-direction property, and the cross axis runs perpendicular to it.
The main axis is defined by flex-direction, which has four possible values: row, row-reverse, column, column-reverse. For row or row-reverse, your main axis will run along the row in the inline direction (row- left to right, row-reverse-right to left). For column or column-reverse it is block direction.
The cross axis runs perpendicular to the main axis, therefore if your flex-direction (main axis) is set to row or row-reverse the cross axis runs down the columns. If your main axis is column or column-reverse then the cross axis runs along the rows.