An EIN number (also known as a Federal Tax Identification Number) is a number used in the United States of America to identify business entities. Most businesses in the US are required to get an EIN when they register and/or start hiring employees.
Many wholesalers, including trusted suppliers listed on the SaleHoo Wholesale Directory require that you have an EIN in order to purchase from them. You can apply for your EIN online.
For more information see http://www.irs.gov/businesses/small/article/0,,id=98350,00.html