The TLD (Top Level Domain) is the last part of a domain (after the last dot).
Usually the TLD is either .com or a country code (e.g. .de)
Example
In the following URL .com is the TLD:
www.aseops.com/DE/
Usually it makes sense to get a country code for TLD. Websites with a country code TLD will be ranked higher in the corresponding national search queries.