Just like elements can have children...

<div>
  <p>I'm a child of the div</p>
</div>

...so can components. Before a component can accept children, though, it needs to know where to put them. We do this with the <slot> element. Put this inside Box.svelte:

<div class="box">
  <slot></slot>
</div>

You can now put things in the box:

<Box>
  <h2>Hello!</h2>
  <p>This is a box. It can contain anything.</p>
</Box>