What's the difference between Description and Full description on a product?

You can use both. They are for different amounts of text.

• Description: the main text near the top of the product form. Use it for a short summary of the product.
• Full description: click + Add full description to open it. Use it for longer details such as features, materials and care instructions. You can write it as text or design it with the builder.

The full description builder only designs the description section. To change the layout of the whole product page, use Settings → Appearance → Product Templates.

Click Save after editing either one.