QuickBooks DataServiceUser is a collection of programs that provides secure database management by limiting direct access and running the database as a separate Windows service when in multi-user mode. It is important because it allows businesses to track daily transactions, run in single or multi-user mode, and provides a separate service to manage the database. The QuickBooks DataServiceUser account is automatically created to manage the database when in multi-user mode and does not pose security risks as it has limited permissions without administrative privileges.