Definition
A distributor is a company that buys products from many vendors and sells them to resellers and service providers, not usually to end customers. Distributors handle logistics, credit, billing and ordering. Many also offer training, marketing help and recruitment of new resellers for vendors.
Vendors use distributors to reach a large number of smaller partners without managing each one directly. The distributor carries the credit risk and the day-to-day ordering work.
This setup is often called a two-tier model. The vendor sells to the distributor, the distributor sells to the reseller, and the reseller sells to the customer. Some distributors also run their own marketplaces and services.
In practice
A new security vendor signs with a distributor so hundreds of existing resellers can order its product through accounts they already have.