An Employer Identification Number (EIN) is used to identify a business entity. You can apply for an EIN (also known as a Federal Tax Identification Number) online here. This is a free service from the Internal Revenue Service which will give you your EIN immediately.
If you’re not based in the US, you’ll also need to get a US mailing address (unless you’re shipping to FBA, in which you can use the Amazon address as your consignee address).
If you call the IRS to get the EIN, ask for a confirmation email/fax to prove you are associated with the EIN. If you don’t ask directly, they’ll put the form in the mail and it could take several weeks to get to you, which is problematic because you need the EIN as part of your shipping documents.