A reseller plan allows you to offer hosting solutions to clients, friends, etc. You are reserved the specifications which is offered in your monthly plan (100GB Disk Space, 100 Accounts, etc). Normally you are given options such as cPanel or Plesk as a control panel option, but the server is shared with others.
Once you find that your company/services is rapidly growing you can then look into getting a VPS/Cloud Server and eventually a Dedicated Server to allow your clients to maximize all speeds (network, hard drive.). Root access is not given on the reseller, but can be an excellent option due to the hosting provider being responsible for that shared server incase problems arise.
Thank you and I hope this helps.