What exactly does aggressive mean?

My husband's large cell lymphomia is in the bone.  When the grade is aggressive,just how aggressive is it -- how rapidly does it spread as a rule of thumb, if there is one.  He is having two new areas of pain not in the areas of where lymphoma was found 6 wks ago.  Could it be moving this fast?  He is to have another MRI this Friday.  something is impinging on nerves in his neck and causing severe pain in his l. shoulder and now down the arm.