Imagine a namespace as a drawer in which you can put all kinds of things: a pencil, a ruler, a piece of paper and so forth. These are your belongings. Directly underneath your drawer is someone else’s, and he puts the same things in it. To avoid using each other’s items, you decide to label the drawers so it’s clear what belongs to whom.
Previously, developers had to use underscores in their classes, functions and constants to separate code bases. That’s equivalent to labeling each others belongings and putting them in one big drawer. Sure, it’s at least some kind of organization, but it’s very inefficient.