What is a Pending status? 

A migration in Pending state is a migration that is not completed.
The migration platform detected a throttling or too many timeouts at the source or at the target and stopped the migration.

The migration will restart automatically later where it stopped.
There will be no duplicate items: like a delta pass, it restarts exactly where it stopped and processes only non-migrated items.

Google throttling

When Google is used as a source, it uses throttling to limit the the amount of data downloaded (read) to between 2.5 and 3.5 GB per day (see http://kb.cloudiway.com/google-throttling-limitations/ ).
Cloudiway migration platforms takes care of this and automatically stops when the limit is reached and sets the status to pending. The migration restarts automatically 18-24 hours later where it stopped.

Office 365/Exchange throttling

Please bear in mind that Office 365 can heavily throttle users. When you perform too many calls, Office 365 begins throttling and decreases the number of calls that can be performed each minute, thus reducing the migration throughput. Cloudiway constantly attempts to work at the maximum capacity allowed by Office 365 and is able to achieve excellent throughput. The migration restarts automatically between around 5 and 15 minutes.

Exchange On-Premises

If you receive a Pending status when migrating from Exchange On-Premises, it could be related to your Exchange server configuration, your internet speed or your firewall configuration. The migration automatically restarts between around 5 and 15 minutes. If the Pending status continues, please check your local configuration.

What actions can I take?
There is nothing you can do but wait. You shouldn’t try to restart earlier as you will be immediately throttled by the source or the target.