An ICANN accreditation is needed for registrars to register domains directly with registries like Verisign. The accreditation verifies the identity of the company's officers and tries to evaluate their qualifications. More information can be found on ICANN's website: http://www.icann.org/en/resources/registrars/accreditation
It is rather costly to become an ICANN-accredited registrar, although at least two of them started out as resellers before becoming registrars themselves. Being a reseller is the next-best thing some registrars like Go Daddy and eNom can offer.
ICANN Accreditation is a certification given to you by the highest body in the Domain Name industry - Internet Corporation for Assigned Names and Numbers (ICANN). This allows you to sell domain names under your own brand name. It's a detailed process where a company's capabilities and resources are tested and verified before being certified. After all, you literally get to 'operate' a piece of the internet once you're accredited! If you need more details on this topic, we did a webinar last year that should help - www(dot)accreditation(dot)com/demystify-accreditation -- Team LogicBoxes