Top Level Domain is the last part of a URL. In example, the fyi in fsp.fyi is the TLD. This is different to the whole domain name which includes the domain name such fsp.fyi. One thing to note, any TLD that is just 2 characters are country level TLDs. You may have additional restrictions based on the laws of that particular country. TLDs are also 2+ characters. So, for example, you won’t find fsp.a.