The role varies widely. Typically, there is a lot of administrative work required to manage the cadence of an exec team: Examples - collecting tracking metrics ahead of meeting, tracking action items for completion, preparing materials for customer briefings, etc. A chief of staff helps the CEO get higher output by letting them focus on the decisions and not the process.

My understanding: a masculine term for "executive assistant". Or: a technical secretary. Personal project manager.

At my previous company, where raising money and making sense of accounting was a non-trivial, the Chief of Staff was (like the CEO and the CFO) a former investment banker for +10 years. He definitely felt like the #2 of the company (ex-aequo with CTO & CFO) and not an executive assistant.

In practice, he was in charge of internal communication with non CxO people and the CEO, and a lot of things that the CEO had to do that wasn’t direction external relations (typically press): preparing financial documents, coordinating teams with legal, team morale, etc.

Person who manages budgets, day-to-day, organizational specific priorities, tracking higher level objectives, etc. Its basically a high level Program/Project Manager with a horizontal responsibility.