An internal link is a link from one page on your site, to another one. It is important to use a good internal link structure as its improves the visitors experience (i.e they can find related content easier) and also helps to pass link juice between pages on your site. http://www.im-wizard.com/why-you-should-use-a-good-internal-link-structure/
A link to your homepage should also be on everypage, if not nearly everypage, and important pages should be closer to the top of the site tree.
it will be difficult to explain the overall internal linking strategy on this forum. since you are new in SEO you have to research online and take out common points from them. this way you will not only learn linking strategy but also would be able to learn SEO. best of luck....
It's simply interlinking your web pages together, usually by relevancy of the topic or page. Mostly, web marketers use footer links to interlink relevant pages.
internal link strategy is the way you have linked the pages of your website to each other. You should link internal pages of your website from relevant pages and try to link to each of your pages from at least another one. Also its highly recommended that the links of your website to be search engine friendly that search engines can detect and index all pages of your website faster.