This describes a nail coating that has not been properly cured. In a UV gel, it may be that the wrong UV lamp has been used and it has the wrong wavelength for the specific product. Or it may be that the percentage level of monomers in the product is too high for them all to become polymerised.
This situation can lead to the leeching out of the monomers whilst wearing the coating or the monomers being released during any method of removal. This can lead to allergies.