Your skin has millions of sweat glands (eccrine and apocrine glands) that release moisture to the surface. If a pore or hair follicle becomes obstructed by dead skin cells, oil, or bacteria, the sweat gets trapped beneath the skin.
A specific type of blockage on the sole of the foot, called a porokeratoma , can feel like walking on a small pebble. Primary Causes and Triggers sweat gland clogged