Looking at the photo: it's the back (dorsal side) of a hand/wrist, and there's a small, raised, whitish/pearly, dome-shaped bump just below the knuckles, sitting along a superficial vein.

Based on what's visible, this looks most consistent with one of these common possibilities:

I can't give a definitive diagnosis from a single photo - things like whether it's tender, whether it's fluid-filled or firm, how fast it appeared, and whether it's changed in size all matter a lot. A few questions that would narrow it down: Did it appear suddenly (suggests blister/bite) or grow slowly (suggests cyst/wart)? Is it painful or itchy? Does it feel soft/fluid-filled when pressed, or firm?

If it's new, growing, painful, draining fluid or pus, or not resolving in a week or two, it's worth having a doctor or dermatologist look at it in person, since photo-based assessment can't rule out infection or other causes that need direct examination.