YouTube displaying a “persistent bar” of “videos from active playlist” at the bottom of the page. “When you click a video from your subscriptions, the bar lists other recent uploads. Clicking your favorite videos, populates the bar with rest all videos. The bar can be useful if you add videos to queue, a temporary playlist built dynamically.”
However, this could be a little annoying: “Even if the cursor isn’t over the bar, it still pops up, so if you go rate a video, favorite or share it, as soon as you move near the bar, it pops up, blocking those settings, so you’ve to either scroll down or click on bar to collapse it. It looks like the spot where the bar is triggered is just above where video player ends when it’s in shrink mode. Once you collapse the bar, it no longer auto-expands, until you watch a new video,” explain Alex.
Source: YouTube Official Blog