What Is The Need Of Biopsy For Mucus Problem?

My father (age 52) is suffering from mucous problem. When visited to a physician, he suggested for blood test, whole abdomen Ultrasound and then stool test (tTG-IgA). All the test are fine except tTG which value is 45. Doctor suggested for endoscopy and D2 biopsy after one week. He recommended some medicine for one week.
Should we worry about any problem in small intestine? Why doctor suggested for Biopsy and what thing will be used for Biopsy?