flex-box in CSS is a tool used to efficiently structure web pages.
flex-box always has a main axis (by default, it is a horizontal axis), and we can rely on this main axis to operate other attributes in a reasonable way.
To effectively apply flex-box, we need to know the following CSS attributes:
flex-start
center
flex-end
space-evenly
space-around
space-between
flex-start
center
flex-end