Docs, what's a swelling on my left testicle indicate?

Need to be examined. I don't know what you mean by "on my left testicle".If you are referring to any intrascrotal swelling it could mean hydrocele or spermatocele or epididymitis or testicular tumor and the list goes on. It would be best if a doctor examined you than for a doctor online to guess. Good luck.Please see a dovtor.
See your doctor. Testicular lumps can be cancer or benign. You don't want to take a chance of misdiagnosing yourself. Testicular cancer can be extremely aggressive so don't delay. Go see your doctor for a thorough exam. S/he may order an ultrasound to determine if you have a mass inside the testicle, but hopefully it is something benign such as a cyst or varicocele (dilated blood vessels). Good luck!