I found breast lump mammogram shows fibroadenoma but I read in the report axillary lymphnodes mildly enlarged and mostly reactive what does this mean?

Don't worry. At 19 it is most likely a benign fibroadenoma , so are reactive lymph nodes frequently seen in young woman, best options to take it out , or get a needle biopsy to establish diagnosis , then if it is fibroadenoma could be safely observed.
Lymph nodes. Are commonly seen on mammograms. It sounds like it is nothing to worry about. Discuss it with your doctor if you are worried.
Swollen glands. The lymph nodes are cell destructive centers and respond to areas of inflammation and cellular mobilization to fight off a discerned problem. Your body's defense system has many types of white blood cells that when their job is completed are destroyed in the lymph nodes.
Lymph nodes. Can enlarge in response (therefore "reactive") to infection or inflammation. Or it can become enlarged with malignancy, such as lymphoma or metastatic cancer. I don't think findings on your mammogram are alarming. Ask your doctor to make sure.