Main slider is a place where the three last added galleries are displayed. It's the default setting which is turned on when the first time you log in to your User Panel. In every template you will find it at the top of your site, just look at example below:
You can also manually set the order of slides on your main slider. To do it, go to 'Pages' and click 'Home Page'
Now you can see button 'Add slide'. Click on it and enter the slide title, decide whether you want to show one of your galleries on slide or not, type the name that should be displayed on link button and add picture.
You can add as many photos as you want and link to each gallery on your portfolio, create slider with favourite photos or you can show there only the most popular galleries.