I have a grape size lump on my left testicle it hurts nothing

Depending : Depending on how long this lump has been present, it could be many things. Most likely it is a spermatocele or an epididymal cyst, which are benign conditions but may be bothersome enough as to warrant treatment. All lumps and bumps in the scrotum should be evaluated by your doctor, sometimes by an ultrasound as well, to make sure it isn't something more serious.

Its : Its important for you to have this evaluated by a doctor. From the description alone it is impossible to tell if this is something that can be watched conservatively or something potentially dangerous.