A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet after the root domain. The ".com" ".org" ".me" you see at the end of links are all considered top-level domains.
For example, in the domain name www.example.com., the top-level domain is .com.
Not all characters are able to be used as a top level domain. There is a list of all TLDs in existence here. Additionally, not all TLDs are offered through Bitly's Complementary Domain program so if there's one you'd like that isn't offered you are recommended to purchase it directly from a domain provider.