Asking your customer for additional information
From Spiffy Stores Knowledge Base
When you update the content of a cart or click the checkout button on a standard cart.liquid page, the store code will look for two special parameters called note and attributes
Adding a Text Note
The note parameter is the easiest to use. You can use this to add a free-form text note to the cart before checkout.
For example, if you are running a gift shop, you may want to ask your customers for a gift message to be included in the package.
Add following html/liquid code to your cart.liquid before the </form> tag:
<p> <label for="note">Gift note:</label><br/> <textarea name="note" id="note" rows="2" cols="60" style="width: 100%">{{cart.note}}</textarea> </p>
This will produce a cart page that looks something like this: