An internal link is a link from one part of your website to another part of your website.
The main menu on your website is probably a good place to find internal links to other pages on your website because typically, all the links in your main menu will point to internal pages.
These internal links are used so that people can easily find their way to important nuggets of information buried deep in your site.
Google also looks at these internal links to try to understand which parts of your website you believe to be most important. So, try not to have a link to your Terms & Conditions in the footer of every page on your website as this might lead Google to believe it’s the most important page on your site because you’re always linking to it!