Flexbox enables alignment of items on the main or cross axis, thus providing a high level of control over the size and alignment of a group of items.
Source: https://www.w3.org/TR/css-flexbox-1/images/flex-direction-terms.svg
Note: the graph above is set for a "row" direction. If the direction is "column", the graph will be rotated 90 degrees, having a main axis in the vertical direction, and a cross axis in the horizontal direction.
To sum up: the main axis is the flow direction we have specified for the flex container. The cross axis is perpendicular to the main axis.
Source: https://www.zachgollwitzer.com/posts/2021/fullstack-developer-series/10-flexbox-crash-course/