“Slider” position is special sidebar for Media slider. Just add one of the available widgets for sliders or any of other widgets. The Demo main slider is made via “TT WordPress Slider” widget in Slider position.
It included the posts of “Front page slides” category. This means, to edit the information in the slider, need to edit the posts in this category. Also, you can create your own category of posts and select to display in the slider.