An invoice ID is a unique alphanumeric identifier that is associated with your purchase. After you complete the checkout process, you'll receive an email with all of your order's details, including the invoice ID, that will look something like this:
You will also be able to find your invoice ID and the rest of your order's details, after you complete the checkout process, on the top (top-left if you're on a web browser) of the invoice page, which looks something like this:
If you're making a purchase without an account, please save this ID as we'll need it to locate your order in case you submit a support request. If you have an account, you will also find all of your order's details, including the invoice ID, under the "My products" section of your account.