If you have started using Vtiger CRM post December 11th, then refer to 'How to set up alerts for idle records?' FAQ.
The red circle beside the Opportunity indicates that the record has been idle for a specified duration of time.
There has been no contact made with the Opportunity either through Phone, email, SMS, events, etc.
The red circle depends on the Idle record notifier settings as in the Notification Center.
You can define the duration after which the record will be marked as Idle, by following these steps
1. Click on the Bell
icon on the right top of your Vtiger page.
2. Click on Settings
3. Click on the dropdown icon besides the Opportunity module
4. Enable the switch for Record is Idle For