When a product goes out of stock and cannot be added to cart, you have an option to modify the label of Add to Cart button to show a more descriptive text.
In your admin dashboard, navigate to System > Features. Select B2C from the dropdown.
Locate and enable Show Out of Stock Messages and enter a text in Title for Out of Stock Messages field.
Click Update to save your changes.
This is how it will look on the frontend for products that not in stock: