The SharePoint Migration
Tool authenticates to the destination tenant after which you are prompted for the source file location and destination SharePoint site collection where you want the files to be migrated. After you submit the migration jobs by selecting Migrate, the scanning, packaging, uploading, and importing steps are performed in parallel across all the files provided for migration.
Use a Site Collection administrator account on the target SharePoint site when prompted for credentials.
AUTHENTICATION: After opening the tool, the first thing you must do is authenticate to the destination -- the tenant where you will be migrating your files. Providing your username and password to the tenant associates the migration jobs you submit to this account. This allows you to resume your migration from another computer if needed by logging in with the same credentials. This account should be a site collection administrator of the destination you want to migrate. The following authentication methods are supported:
SAML based claims
Client certificate authentication
If multiple authentication methods, including NTLM or Kerberos, are enabled in the on-premises SharePoint Web Application, NTLM and Kerberos authentication are not supported by the SharePoint Migration Tool. Please use a secondary form of authentication or convert the Web Application to use NTLM and/or Kerberos authentication only.
SCAN: After you select Migrate, a scan will always be performed on every file, even if you decide not to migrate your files (see Advanced Settings). The scan verifies that there is access to the data source and write access to the SharePoint destination. It also scans the files for known potential issues.
PACKAGING: In the packaging stage, a content package is created that contains a manifest.
UPLOAD: In the upload stage, the content package is uploaded to Azure with the manifest. Before a migration job can be accepted from a SharePoint-provided Azure container, the files and manifests are encrypted at rest using the AES-256-CBC standard.
IMPORT: During the import phase, the key is provided to SharePoint SAS. Only Azure services
and SharePoint are interacting to fetch and migrate the data to the destination. This process is a timer job-based but does not prevent other jobs from being queued up. During the import, a report is created in the working folder, and live updates are made. After the migration job is completed, the log is stored in the Azure container and a final report is created.
SESSION AND RESUME: While the migration is being performed, the tool saves information of the session in a hidden list on the user's OneDrive. This allows the migration tool to resume any previous migration sessions.
To know more about SharePoint 2013 to SharePoint online migration