Domain Extension (TLD)  stands for top-level domain and it refers to the very last segment of a domain name (basically what goes immediately after the final "dot" symbol). 

The basic categorization of TLDs goes as follows: 

With the TLD option, you can specify the countries or TLDs you want Dibz to focus on when looking for link building opportunities. For example .co.uk or .com.au limits your search to sites with those extensions. Of course, you can look for opportunities on a global scale via our tool, but you can also specify your targets, if your campaign is only focused on a specific area. 

TLD option is available from the 3rd step of your Dibz search setup. The step called Adjust the Final Details and Parameters of Your Search
​

Related: Where do I finalize the search and fine-tune additional parameters?