In Testing mode your automation campaign is live and will trigger emails based on real events on your website, but the emails will send to the 'Ometria-testing' email address instead of to real contacts.
Test sends don't count towards your account's frequency caps.
You can see your testing activity in the Activity tab of the Campaign Performance: Automation campaigns dashboard.
To set an Active campaign to Testing you must Pause it first.
If you switch an Active campaign to Testing then any contacts who entered the campaign while it was active will continue their journey through the flow, but will not receive emails - these are sent to the 'Ometria-testing' email address instead.
Ometria automatically pauses campaigns which have been in testing mode for three months.