unwrapping:

Tumblr Settings Tuneup: Conversational Notifications
Tumblr added a setting for Conversational Notifications, which is on by default. When you reply or reblog with comments, you become subscribed to other’s post comments (e.g., “tubastank also replied to this post”) in Activity. You can turn this setting off in Notifications Settings. On mobile, you’ll find the setting by tapping the Account/Person icon > Gear icon (upper right) > General settings > Notifications > Conversational notes.

If you leave Conversational Notifications on, you can toggle notifications per post (silencing the busier ones) by viewing the notes. Click or tap the lighting bolt icon in the upper right of the notes. 

On the desktop, a white bolt on a blue circle means notifications are on (subscribed) and a blue bolt with a white circle means notifications are off (unsubscribed). On mobile, when you tap the bolt, you’ll see a prompt telling you if conversational notifications are now subscribed or unsubscribed.