(from lunarmimi's braid tutorial)
braids are difficult to explain but this is a good visual breakdown of what a basic braid looks like!
there's kind of a soft zigzag in the center, since the sections of hair gently twist around each other.
i find that really lengthy explanations end up confusing more than helping, i'd say it's best to let visual diagrams speak for themselves.
this one is a good visual for how they look when they twist, too. same structure, just a different angle.
you can see how the hair sections loop together in an over-under way here, although this gif does go pretty quickly. i'd recommend watching how-to videos on YT for a better look.
edit: added a second diagram and a braiding gif