Somatosensation is a pretty big word! Simply, it refers to the sense of touch. When you move your hands over something like a lego piece, you're activating mechanoreceptors in your hands that connect to your brain and give the sensation of feeling the lego's bumpy surface.
So how does somatosensation work?
In your hands, there are many types of mechanoreceptors that detect all kinds of tactile stimuli. There are Meissner's corpuscle that sense fluttering vibrations, Merkel cells that feel bumps and indentations, Ruffini endings that detect stretching, and even Pacinian corpuscles that detect very strong vibrations.
These mechanoreceptors lie in the epidermis and dermis layers of the skin (outermost and middle layer, respectively). When these mechanoreceptors are activated, they send electrical messages to the brain, creating the touch perception. There is an area of the brain called the somatosensory cortex which is where somatosensation for every point on the body gets sent to. The coolest thing about the somatosensory cortex is that it is essentially a map of the body! This map is referred to as the somatosensory homunculus.
So when you feel a soft piece of clay or a fuzzy pipe cleaner, remember that (as always) there is a lot going on in the nervous system!