From Halopedia, the Halo wiki
It is represented by a tomb stone with an empty grave.
There is a skull in each mission. In order to collect a skull, you have to complete the optional objective, which is about killing a specific number of an enemy unit type. After doing that, a location on the mini-map will flash shortly - keep an eye on this, as this is where you will find the skull.
- It is a possible reference to the "Grave Robber" Achievement in Call of Duty: World at War for finding all the death cards. Death cards also function extremely similarly to skulls; they make the game harder or easier to beat, and can add some interesting gameplay effects.
- Although there are 17 skulls to obtain, the skulls for the Elite Honor Guard Wraith, and for the Flame Decal Warthog do not count towards the achievement.
- It probably refers to the player collecting skulls, and to collect a skull, normally you have to rob it from a grave.