Phimosis is a vague term. In common usage it refers to any condition in which the foreskin cannot be retracted because it is too tight. But true phimosis should be distinguished from a “normal” tight foreskin!
Phimosis consists of a thin band or ring of connective tissue that is embedded towards the front of the foreskin. This (often whitish) ring narrows the opening, which makes it impossible to retract the foreskin. The tip of the foreskin has lost its normal suppleness and is scarred.
You can treat this problem by using clobetasol cream (Dermatovate 0,05%; for more information). If this does not work you have a choice of preputial plasty or circumcision. The technique of preputial plasty is a limited dorsal slit, which leaves the foreskin intact (for more information).
With a tight foreskin of phimosis a circumcision might be recommended, while the problem could have been solved otherwise. If you value your foreskin, ask for a second opinion from an expert before having your foreskin removed.