A second-level domain (SLD) is the portion of the domain name that is located immediately to the left of the dot and domain name extension. Example 1: The SLD in coolexample.com is coolexample. Example 2: The SLD in coolexample.co.uk is still coolexample. You define the SLD when you register a domain name.
A country code second-level domain (ccSLD) is a domain name class that many country code top-level domain (ccTLD) registries implement. The ccSLD portion of the domain name is located between the ccTLD and the SLD. Example: The ccSLD in coolexample.co.uk is .co.
See What are top-level domains (TLD) and country code top-level domains (ccTLD)? for more informtion.