How to send hidden data from product page to the cart and where the admin views orders

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I am new to shopify and am having trouble figuring out how to send a hidden string of hex code from my product page to the cart (with the corresponding product) and also to where the admin views orders. 

Right now, I just have a hidden field on my form and put the hex codes in there, but I don't know where that information goes after the form is submitted (when the "add to cart" button is pressed)