Halo 3...skull?

Yeah, I will say hat I am a noob on Halo (I got the 3rd one a few weeks ago, and I have been working up from Easy), and have no other Halo games. So I just started playing on Normal like a week ago and i am up to the "Cortana" level (god I hate that level), so last night I played "Covenant", and I noticed this skull thing on the second tower, the one where you aren't supposed to land. Can anyone explain to me what it is?
Plz be gentle if I sound like a noob.
Plz be gentle if I sound like a noob.