How does CSS Flexbox work and what problems does it solve?

Flexbox is like an adjustable shelf organizer. You decide whether to arrange items side by side (row) or stacked (column), how to spread them out (space-between, space-around), and how to line them up vertically within the shelf (align-items). Each item can also control its own size with flex-grow — greedy items can grow to fill leftover space while others stay their natural size.