I have a lump at the back left side of my neck,what does this mean?

Depends: This could be cyst, mole or any lymph nodes. Please see a doctor to get it checked.

Many things: Palpatory lumps at the neck can be due to enlarged lymph nodes, thyroid nodules, lipoma, abscess, and other etiologies. Painless, nonmobile enlargement of the lymph nodes as a warning sign of underlying occult malignancy. Examination and evaluation by a physician is recommended to ruled out serious underlying conditions (e.g. lymphoma, neoplasm).

Many: It could be one of many things. No point anyone guessing online. Please get examined by a doctor. Good luck.