A slider is an icon that allows a user to reposition what is displayed. They only show up when a picture, text, etc. is too wide or tall to entirely fit on a screen. For example, there is almost certainly a slider on the right side of your browser while you peruse HC.
Normally, an application allows the browser to provide the slider. That's because a browser does it better than an application inside the browser. Unfortunately, the people who wrote vBulletin must have decided that they can do it better than the browser as the default setting seems to be for vBulletin to provide them when pictures are too large to display. More unfortunately, those people are wrong. When you have a series of pictures, to scroll back and forth you have to use the browser slider to go lower in the message, use the vBulletin slider to reposition the picture that you can no longer see and then use the browser slider to go back to the pictures. It's a real PITA.
The good news is that there seems to be a way to turn this "feature" off. Someone did it after the last update. I sure hope that person or someone else can do it again.
Edit: I just saw Salty's post. I have a 23" monitor and it's not near large enough. I doubt that there is a monitor large enough to handle some of the posts in the photography thread. Of course, having vBulletin do vertical scrolling isn't as painful as having it do horizontal scrolling.