"Is this erotic?", it is not so much a question as it is a statement that the illustration contains an underlying eroticism despite an otherwise tame subject matter. It also is often used for illustrations that are humorous and the joke involved is sexual (or perhaps it is a double-entendre). Basically, this tag is for when the image is not explicitly adult, but suggests an underlying erotic tone none the less.
Similar English phrases might be "Strangely erotic" or "Strangely arousing".
This tag is NOT meant for images rated R-18. It sometimes serves as a warning that images may not be safe for public viewing despite lacking the R-18 rating. It is also quite often used to indicate a sexual joke is present in the image.