Shortcode enable wordpress plugin developer to create special content (form,gallery,slideshow, etc) that users can easly attach to certain page/post.
Mostly shortcode was inserted into post/page. But actually we can also call shortcode within our theme directly. For example if you have a shortcode to call a gallery and you want the gallery to be displayed on top of your web page / landing page.
Insert a shortcode to theme is a bit tricky. Calling the shorcode is easy, the problem is where to put the shortcode. To properly insert a shorcode to desired position in your theme need a little HTML knowledge.