I cant understand what this DRC error means.
Especially I do not understand this box. Does it mean that the Soldermask will cover part of the pad? How to correct it?
It's due to the fact that your reference designator font is "proportional", not "vector".
Proportional fonts in eagle are collision detected with a rectangular box which is what the DRC is erroring about - the bounding box of the ref-des intersects the stop mask.
Vector font converts to a series of lines so collision detects the actual lines rather than the bounding box.
I'd recommend always using vector font. In fact there is a setting in the layout editor options for converting all fonts to vector.