What is reseller access within WHM?
A reseller is a cPanel account that has the ability to create other cPanel accounts. Because these resellers can create cPanel accounts, they have the ability to "resell" cPanel hosting.
When you sign up for a VPS Hosting plan or a Dedicated Hosting plan, you have reseller WHM access. This is why you have the ability to create separate cPanel accounts for your users / customers.
How do resellers manage other cPanel accounts?
Resellers manage cPanel accounts using a limited WHM interface. Resellers have WHM access, but they don't have all features that WHM has to offer (for security reasons).