Users can subscribe to campaign with the purpose of receiving notification for new ideas and comments.
Subscribing to Campaign allows the subscriber to receive notification for all the new ideas and comments for the only campaign subscribed incase under profile notification 'New ideas' has 'Disabled' selected.
Steps to subscribe/ unsubscribe campaign.
1) Click on the desired campaign. You will see Subscribe to campaign link just below the Campaign Question / Challenge Statement. Click on this link to enable idea/comment notification for a specific campaign.
2) Subscribe campaign is like toggle switch .Once the user subscribes to a campaign, they would see the Unsubscribe from campaign option. Click on unsubscribe link to stop idea/comment notification for a particular campaign.
Administrator has ability to enable or disable subscribe campaign link from user. This option can be set from Community settings >> Basic settings >> Manage campaigns.
Help article on campaign settings: