Reseller
Reseller is a panel that sells SMM services it does not produce, buying each order from another panel through that panel's API and charging its own customers its own rate.
The mechanics are simple. The reseller connects to an upstream panel with an API key, imports the service list, sets its own name and price for each service, and forwards every order it receives. When a customer orders, the reseller's panel places the same order upstream with the same link and quantity, pays for it from its balance there, and copies the status back. The reseller keeps the difference between the rate it charges and the rate it pays.
For a buyer, the practical effect is distance. Delivery does not change because a reseller is in the middle: the accounts, the speed and the drops come from the same supplier. What changes is the price, which carries every margin in the chain, and the time it takes to fix a problem. A refill request or a stuck order has to travel upstream one panel at a time, and each panel can only act on what the one above it reports.
Resellers can add something useful: payment methods the upstream panel does not accept, support in another language, or a tidier catalogue. They can also rename services, so a service sold as non-drop by a reseller may be an ordinary service upstream. The reseller's own service description and refill terms are what you can hold it to, whatever the panel above it promises or does not.
What it does not mean
Reseller is not a quality label. A reseller buying from a good supplier can deliver better results than a panel running weak supply of its own. The word tells you where the order goes, not what arrives.