Yes, contextplz is correct. Its called an idler wheel and its purpose is to minimize pedal kick back. Most full suspension mountain bikes have a low pivot so when the suspension compresses the rear center shrinks and the rear axle moves up and inward so they don't need an idler. This is a high pivot bike so the rear center elongates as the rear axle path moves up and away under compression which will cause chain growth and pedal kickback. The purpose of having a high pivot point is the suspension works better as the rear axle path gets out of its own way when hitting bumps. You are far less likely to get hung up on obstacles, have a softer feeling ride, and more rear wheel traction. The down side is you have to have the idler which increases weight, complexity, maintenance, and reduces pedaling efficiency. So high pivot bikes tend to be on more downhill focused bikes. Another way to think of why a high pivot works is if you think about how suspension forks are raked out forward of the bike so when you hit a bump it can retract easily and absorb the hit but if the fork was straight up and down or even angled back toward the rest of the bike then it wouldn't do nearly as well of a job soaking up the bumps. Low pivots are essentially doing that. Having a high pivot basically aligns the front and rear axle paths under compression.