Checkend Sample Product
This is the description under Tab #1.
You precede it with a "Heading" tag. In this case, it's a Heading 4 tag (in HTML code).
You can add links here, images, and even videos. Pretty much anything that you can add from within the Shopify product editor.
This is the description under Tab #2.
Ingredients are likely specific to product and should be placed within each product's description.