Every product page informs you whether the product is currently available for purchase or out of stock.

If you see a "Buy Now" or "Add to cart" button on the product page, it means the product is currently available for purchase.

If you see "Learn more" or "Get Stock Alert" button, it means the product is currently out of stock. You can still submit your email to receive update once the product is back in stock. When the product is back in stock, this button will be changed to a "Buy now" button.