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You really can't tell what it is without an examination.
It can be a hydrocele, a collection of fluid; or varicocele, a collection of enalrged veins; an epididymal cyst, a fluid-filled sac; or spermatocele, a sac filled with sperm.
There's also the possibility that it may be a simple boil, an infection that gets into the body through a pore or hair follicle. Treatment may be to open and drain it and apply antibiotic ointment. Depending on the size, it may be necessary to pack it with sterile gauze that is changed daily to allow it to heal from the inside out.
Testicular cancer is a long shot, since that doesn't just suddenly present as something you can feel overnight.
You will need to go in for an exam to learn what it is.