Data onboarding is the practice of transferring offline data for online marketing purposes. Data onboarding is usually done through DMP.
Data onboarding is mainly used to target online an offline segment of audience. An offline dataset of customer records is used to retrieve the same customers in an online audience. It allows to connect offline data to online users.
For more details see CRM retargeting.
Note that in traditional marketing data onboarding may have a different meaning.