A contact is a person who interacts with a brand across one or more channels, such as email, mobile, web, etc.
A contact is identified by a contact key, which is usually the same as the subscriber key in Marketing Cloud or the contact or lead ID in Sales Cloud. Whenusers register with the mobile app, they will create new contacts in Marketing Cloud with their device IDs as contact keys. However, these contacts will not be associated with their contact keys in Sales Cloud until they are synchronized through Marketing Cloud Connect, which can take up to 24 hours. Therefore, newly registered users will create additional contacts in Marketing Cloud until they are matched with their existing contacts in Sales Cloud. References:
https://help.salesforce.com/articleView?id=sf.mc_co_what_is_a_contact.htm&type=5https://help.salesforce.com