A remote move migration moves mail from an on-premises Exchange Server (2010 or higher) to Exchange Online. It can also be used in a hybrid deployment, to move mail between the on-premises Exchange and Exchange Online, in either direction. You'll need to enable the MRS Proxy on the on-premises Exchange Server.
Mailboxes moved in this way are specified in migration batches. You will use a CSV file to identify the specific mailboxes to be moved.