The new Firefight mode in Halo 3 ODST may seem like nothing more than Bungie's version of Gears of War's Horde mode, but the dots in the top left corner... they are what's different.
In Halo 3 ODST, the new Firefight mode will allow four players to fight against waves of Covenant enemies. Levels are set in environments pulled from or based on sections of the ODST campaign. They are unlocked as the player progresses through the campaign.
A box in the upper left of the player's field of view represents the progress of the player team's squad through their Firefight mission. It shows how long they've survived in the relatively complex progression of the Firefight mode.
Each slash mark in that box represents a wave of enemies. Each new wave — until the final one of a round — is randomly generated. Enemies could be annoying grunts or an assaulting Wraith. But the fifth round will also be brute chieftans.
Five waves completes a round, represented by a dot. Completing a round activates rule-tweaking skulls, similar to those discoverable in Halo 3. The more rounds completed, the more skulls are live. The end of a round also spawns some health packs and ammo upgrades. (ODSTs don't have regenerating health like Master Chief; they do need health packs).
Every three rounds that are completed finishes a set, which is indicated by a number to the left of those dots. Bungie won't say what happens when the player completes a set, but promises it'll be cool.