It’s the Auto Ripple button. The icon looks like two small rectangles, one gray and one green with an arrow on top. It’s at the very top where most of the icons are.
It’s a really important feature of Vegas, and one I use more than anything else. It can be very helpful and if you’re not familiar with it, it can mess you up faster than anything else by sliding clips over each other you don’t want to slide.
Keep in mind that there are several settings for it as well. One moves all the tracks to the left and the other moves only a selected track. Play with it and you’ll figure it out.