What is it?
Does anybody knows what is that line in DSS2 Blue?
What is explaining of its appearing?
That's satellite or asteroid trail. You'll see more of them, they're quite common.
by Vinokurov moderator
Unfortunately, I can't see the image just now. If you are seeing anything weird in the DSS images, they are probably #artifacts of the photographic plates used for those images. Following the #artifact hashtag will show you other examples, which I hope helps to answer your question.
Taking data with a telescope is not perfect, sometimes you can also see dust motes, in the discussion boards at Common Artifacts you can find more examples. Like here: http://talk.diskdetective.org/#/boards/BWI0000009/discussions/DWI00000ot