Can anyone explain to me what this tag means ? and why is it badly viewed ?

Tracing is when an artist takes an image (real life picture or another mango work), and then just copies it so blatantly they might as well have just placed their paper over the original picture and traced its lines. Of course, some things are altered, face, hair, that sort of thing. For example:
https://twitpic.com/9nyy0t
https://twitpic.com/9nys74
https://twitpic.com/9nxzxy

Just to be clear, it is not a taboo method or anything, and it is actually often used for things such as landscapes and scenery in general. But when an artist uses it for characters, it is mostly viewed as crossing the line.

In US comics it's called "swiping," and usually applies to artists copying poses, compositions, etc. from other comics, but also can be applied to overuse of photo-reference. Opinions vary (inside the industry and among fans) about how grave an offense (if it is one) swiping is. As the name implies, it can be seen as a form of, if not outright theft of intellectual property, at least a dubious sort of "taking."

There is a line between using photos and other images to make sure you're drawing things accurately (which is good) and just being too lazy to draw things yourself (which is not good), but nobody has ever been very precise about where that line is located (when people notice the swiping at all).

I don't know much at all about how it's seen in the context of manga.

I don't know much at all about how it's seen in the context of manga.

Negatively, because of the copyright infringement issues. Using images for reference (which in the case of "Prism" Shou admitted to doing, but the final verdict was that he went beyond this) is fine, but actual tracing of the images (blatantly copying it, sometimes by actually tracing the lines) infringes on copyright of the material used (hence why "Prism" was cancelled).

My bad I thought its another series.

Oh i see , what a shame it was cancelled , pretty good manga nonetheles, thank you for the clarification