Holes appear in all three games of the Run series. In fact, the only level that technically doesn't have them is Level 10 (At first it doesn't have them, but after you crumble the tiles, there will be holes). You can consider them as a transparent tile (or ghost tile, since you cannot touch them), and through them the player can see the starry background.
Holes are the absence of tiles, so are they considered a tile? Yes, actually. In Run 1 and Run 2, they're like a "walk on it and die" type of tile. However, in Run 3, when you touch them, you float out into space. Unlike Run 1, when you touch a hole that's on a wall, you start floating away from the tunnel. However, if you time your jump right, you may be able to get back inside the tunnel. If you do that after flipping upside-down, you'll get the A Breath of Fresh Nothing achievement.
- In the Level Editor, the eraser is similar to having the role of the Hole tile, since it can remove tiles and replace them with this blank tile.