The onboarding doc is a reference guide as you join the company. This document is literally designed for you, outlining your role + team, people to meet, and how you can get started. Generally, your manager will provide an onboarding doc for you, but you should feel free to augment or create your own if the doc is lacking. This is a great place to collaborate and deepen your relationship with your manager -- point out further areas of discussion.
Components of a good onboarding doc:
Team charter (translated down from the company mission)
Partner teams + people
Expectations + milestones
Project ideas with t-shirt sizing
Engineering rubric or career matrix for your role, if one exists
Reference question: What should be part of an onboarding doc?