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Here is my understanding:

Recently, a security vulnerability of Lemmy has been exploited by some malicious actors. This lead to some instances going down. The vulnerability has been fixed with version 0.18.2-rc.1 of lemmy-ui. But due to the way Lemmy issues and uses access tokens, the sessions has been invalidated in the database. So, the admins are recommending the users to log out and log back in if they haven't done so after the upgrade to version 0.18.2-rc.1 of lemmy-ui.

But I may be wrong. Perhaps others can provide a more accurate description.

It means that you need to log back in, because the server forced you to disconnect.

Where are you seeing this happen?

Usually, a "session" represents a client being logged into a server. For example, when you log into a website like github, a user session is created and bits of it are stored in cookies in your browser. This is what allows you to continue clicking around the website for a while without having to enter credentials constantly.

A session being invalidated usually means that the system has decided that you are no longer authenticated. For example, github hasn't seen you clicking around the site for 60 minutes and so it logs you out. But there are many reasons why this will happen. If you want to use the site again, you have to log back in.

FYI, this type of security is pretty basic stuff for web development. There's loads of info available if you want to learn more.