Enlarged lump on side of neck can occur due to enlarged lymph gland/node .
Lymph node can get enlarged in infection, inflammatory conditions, an abscess, or cancer.This may also occur in Immune or autoimmune disorders that can cause swollen lymph nodes like rheumatoid arthritis and HIV.
Cancers that can cause swollen glands include leukemia, Hodgkin's disease
Other possibility is of lipoma. This is benign tumor and is harmless.
As such a visit to doctor for clinical checkup and evaluation is necessary.
Your doctor can advice blood test and lymph node biopsy to make a diagnose