1. effectively integrate a new employee or client into an organization
This verb means to help someone transition into new employment, or to successfully integrate that person into a team. As a result, you are unlikely to hear it used in a social context. You might hear a Manager speaking about 'onboarding' new staff, or a Sales team talking about new clients they have just 'onboarded', which means they have just signed them up and begun a business relationship with them.
2. built into or an integral part of
This is an adjective which tells us if something is included with something or is a part of something. You can have an 'onboard radio', 'onboard systems', or an 'onboard radar' - anything at all that is fixed into a device or a vehicle, for example. You will sometimes see this word spelled with a hyphen (on-board), and if it describes something that is built into something else, it means the same thing.