The term 'last seen online app story' is all about the last time a user was present in the app while dealing with the story aspect. Let's say you have a messaging app that also has a story feature. When a user views or uploads a story in that app, the app records that moment as the last time they were online in relation to the story. This can be used for different purposes. For instance, from a social perspective, it can give you an idea of how active your friends are with the story feature. From a marketing point of view, if you're promoting something through stories, it can help you understand when your target audience is most likely to be online to view those stories.
It means the time when a user was last active or online within the context of an app's story feature. For example, if an app has a story section like Snapchat or Instagram, 'last seen online' would be the last time the user was in the app interacting with stories, such as viewing or creating them.