It is often normal that the foreskin of a prepubescent boy does not retract fully. The foreskin is often glued slightly to the glans at birth. The foreskin usually takes off or dilates gradually in the first years of life since the glans gradually becomes bigger. It is simply a physiological phimosis that goes away by itself without treatment. However, it should be resolved before puberty.
A pathological phimosis is a foreskin too tight that no longer retracts for lack of flexibility. Its tightening is variable. It can be light or sometimes very tight which prevents to see the glans completely.