In the Halo universe, the biggest threat is an alien hegemony called the Covenant. Who are they and what do they want?

One of my best recent TV memories was meme-watching Paramount Plus’s Halo TV series ... a lot more actual Covenant fighting this time around, as the focus on the Fall of Reach would suggest ...

The UNSC suffered a major blow in 2552, when the stronghold planet of Reach fell to the Covenant. However, a single vessel named the Pillar of Autumn escaped battle to one of the seven Halo rings ...