Hi, Your subdomain is a unique identifier of your Zendesk account, which appears before .zendesk.com in your account URL. For more details please visit:
https://support.zendesk.com/hc/en-us/articles/4409381383578-Where-can-I-find-my-Zendesk-subdomain
If you have any specific questions feel free to open Zendesk.com and chat with one of your agents.