It's a custom-made animated background for a beatmap. Some of them are kinda subtle, while others are full of motion and can be compared to videos (I've seen a few Touhou and Vocaloid songs that have their storyboards designed to replicate a song's music video). There is also
a single ranked map that uses different gameplay mechanics than the normal game, and relies on the storyboard to work.
If you're looking for performance gain, disabling the storyboard shouldn't have any significant effect. Storyboards are much less intensive than videos are. I personally keep it on, and only disable it per-song only if it gets too distracting.