This is a barricade that has not been damaged.
Barricades are used to stop zombies from entering rooms and some maps (Farmhouse, Airbase, Karnivale). You can't walk through them when they are completely broken they also can't be upgraded. You can fix barricades by pressing the F key, and it awards you XP for doing so.
- 10-20 XP for light damage
- 30-40 XP for moderate damage
- 50 XP heavy damage
- 60 XP for completely gone
Keep in mind that there is a cap on how much XP you can earn from fixing barricades.