What does the thin blue line over the Finder icon mean?

Hello everyone!

I have a question about something that I have noticed recently: a thin blue line over the Finder icon in the Dock. I don't know for how long it has been there.

What is weird is that it has nothing to do with my screen malfunctioning as it disapears when I hide the Dock or use an application in Full-Screen mode.

Here is a picture of what it looks like...

Could anyone tell me what the purpose of this line is, please? I know it is probably not a major issue, but as any Mac user, I am fussy about details...

Thanks in advance!

Best regards!