The tax ID number is either your SSN or, as js250 explained, the EIN (employer identification number) given to businesses. These numbers are used by the IRS to identify you. At Eden, we require one or the other, depending on whether your account is paid to a person (SSN) or a business (EIN).
Why does Eden need this number?
Each year, EdenFantasys reports affiliate earnings to the IRS and needs to include a SSN or EIN for people who have been paid commission. Affiliates who are paid $600 or more in commission in one calendar year will be sent a 1099 for their earnings, so they can report them on their taxes. All affiliates who are paid commission, regardless of the amount, are responsible for reporting their earnings on their taxes.