A domain name is split into two key levels: the top level domain (TLD) and the 2nd level domain.
The TLD is the domain extension (.com or .net for example) and the 2nd level domain includes the part you choose (eg thecoderszone.com).
Beyond that, a domain can have up to 127 levels, or subdivisions. For example, dashboard.thecoderszone.com is a 3rd level domain, m.dashboard.thecoderszone.com is a 4th level domain and so on.
Subdomains can be seen as 3rd level domains (they are a division of the 2nd level domain). Likewise a sub-subdomain (if there was such a thing) would be 4th level domains.
Going back to the first example, dashboard.thecoderszone.com is a subdomain of thecoderszone.com, and m.dashboard.thecoderszone.com is therefore a subdomain of thecoderszone.com.