The Forerunners categorized the Technological Advancement of civilizations into Tiers. This system was later adopted by the Covenant. All life and civilizations in the Universe fall into one of the categories. The lower the Tier number, the more advanced the civilization's technology is/was. It should be noted that this is also a reference to Bungie's favorite number of 7.
Tier 7: Pre-Industrial
Tier 7 is one of the most common and stable states, with limited weaponry and environmental threats. Societies tend to be small and scattered, driven by subsistence farming, foraging, or hunter-gathering needs. Technology is limited to simple hand made tools, weapons, or agrarian implements and methods, but a very broad understanding of planetary and solar mechanics is not uncommon.
Tier 6: Industrial Age
Tier 6 is the outset for massive urbanization. Agrarian societies can remain stable in the pre-industrial stage, but Tier 6 population strain and mechanized food production invariably create political and economic pressures very few can balance. Moving past this usually promises advancement. Some societies improve environmental and medical understanding concurrently with mechanical and transport advancement. Those that do not are frequently doomed.
This level is also where the Unggoy were before joining the Covenant.
Humanity entered this Tier in 1781 with the advent of steam technology.
Tier 5: Atomic Age
Tier 5 species usually begin focusing on clean energy production. The occasional belligerent species will use atomic energy for weapons, often resulting in mass extinctions. In-atmosphere craft are a hallmark, often leading to manned space flight, albeit on a short-scale.
Tier 4: Space Age
Tier 4 is often the final resting place for species intelligent enough to break free from their cradle's surface only to fill the gulf surrounding it with war. Their comfort-focused technology can include medical advances.
Humanity reached this level in 1957, when the first artificial satellite was launched.
In some period of their history the Jiralhanae reached this level, but because of a bloody civil war between rival master-packs, their civilization fell back to Tier 7.
Tier 3: Space-Faring
The species in this tier have efficient Slipspace navigation, mass drivers, asynchronous linear-induction weapons, holocrystal storage and semi-sentient AIs (though their creation requires memory transfer from the freshly deceased and/or Flash Cloning). They have had no outside influence.
Humanity stood at this level before the Human-Covenant war. This level is also where the Lekgolo were before joining the Covenant. Humanity reached this tier in 2291, when the Shaw-Fujikawa Translight Engine was created.
Tier 2: Interstellar
The species has the ability to perform exceedingly accurate Slipspace navigation, near-instantaneous communication and man-portable application of energy manipulation. The unified state of the 26th century Covenant stands at this level.
Both San 'Shyuum and Sangheili brought the other races of the Covenant to a high technological level when incorporating them into the Covenant. The two primary extant interstellar civilizations were space-faring prior to discovering Forerunner artifacts - or each other - but they have not successfully reverse-engineered those artifacts.
Humanity entered an earlier level of this tier at some point during the Human-Covenant war. By the end of the war, with warships such as the UNSC Infinity, humanity stands at this tier of knowledge and near-full understanding from the studying of reverse engineered Forerunner and Covenant technologies, continued research in the field, as well as research in the laboratory. Though, due to the remnants and threat of the Covenant military, equipment so far is not as widely mass produced yet with the exception of the Infinity and very few others. This may suggest that humanity may have reached this level of technological advancement c.2557.
Tier 1: World Builders
The species has the ability to manipulate gravitational forces, create AI with full sentience, fabricate super-dense materials, perform super-accurate Slipspace navigation, the ability to create life, and the ability to create worlds (e.g.; Onyx, the Halo Array and The Ark).
If Humanity retrieves the second piece of the Janus Key and locates the Forerunner artifacts, they may be able to reach and control this tier.
Tier 0: Transsentient
As the Forerunners had no examples of civilizations with technological accomplishments greater than themselves - with the exception of the Precursors - this is a theoretical ceiling. It is suspected that they can travel across galaxies and accelerate the evolution of intelligent life. Along with this the use of neural physics is suggested at this stage.