Here is an awesome interactive article by Josh W Comeau on flexbox. It has some good examples in there on what you’re talking about too.
In many situations, the default flex-direction: row (elements laid out left to right) is what you want when using flex.
flex-direction: column on the other hand lays elements out from top to bottom. This is basically how elements are laid out anyways—that’s why it’s less common to use this direction. You would typically use this direction on a container that has a larger height than its child elements because now the flex properties can actually position elements within the container and utilize the empty space. Otherwise, most flex properties don’t do anything in this direction.