When we are on a user's page, what does the "Last seen" information mean? Does this information indicate that the user (profile owner) logged into the system? Or that any other user saw something from this user? Or does it indicate neither of these things, but something else?
I could not quickly find a definitive answer to your question. Certain actions are supposed to update the "last seen" field, but I am not quite sure which ones exactly. Judging by the fact that my profile says I was last seen 5 minutes ago and I haven't posted anything recently (before this post of course), browsing while being logged in probably triggers an update. Probably logging in also triggers an update.
What I do know is that posting a question or answer is supposed to update "last seen". Note that these updates have their own quirks.
Furthermore the "last seen" field refers to activity on a particular site (mathematica.stackexchange.com) rather than stackexchange in general (you can have a different "last seen" time on math.stackexchange.com simultaneously). Source: Question about the 'last seen' field.
I would say that if somebody else views a post of yours, this does not trigger an update to your "last seen". You can use "last seen" to see when a user was last active, if others can update your "last seen" this would not be possible.