My mother has a bun of 53, creatinine 1.81. bun/creating ratio is 27. electrolytes normal. What does all of this mean?

Kidney concern. Chances are that Mom's kidneys aren't working as well as they should. If she's elderly, this may just be from old infections or high blood pressure, but she'd do well to comply with her physician's guidance. This isn't a great danger to life or an assurance of serious disease to come, and it may simply be the effects of medication -- ibuprofen is famous. But it does need evaluation.