What does a benign high density lesion of 5mm in the right breast on mammography indicate? physically no lumps observed. this is for my mother aged 40

Probably benign: I presume from your question that the radiologist reading the mammogram felt that the density likely was benign. You would not expect to feel a 5mm lesion unless it was just beneath the skin in the breast. If the density looks benign but is a new finding, a followup mammogram should be done in 4 to 6 months to be sure it is not growing. If there is any concern at all, it should be biopsied.

Probably nothing.: What is concerning is that "high density" is not usually used to describe a benign lesion which are more often described as low density. A lesion may be called benign if it has well defined borders and has been stable for years, or has been determined to be a "simple cyst" by ultrasound. A 5 mm lesion would not be palpable (able to be felt) unless it were near the surface since it is so small.

Uncertain without: more details, but if the radiologist made a specific diagnosis of benign(BIRADS category 2), then no further follow up is necessary. There are a number of benign entities that can occur in the breast, ask your doc or the radiologist what exactly it is.