The Top Level Domain (TLD) is (as its name says) the hierarchical highest domain.
It's the end of the domain name.
Every TLD is managed by a certain authority.
There are Country Code Top Level Domains (ccTLD) like .uk, .us, .de, .ch, .at, etc and Generic Top Level Domains (gTLD) like .com, .net, .org, etc.
You can choose the TLD to query in your configurations.
List of all TLD