phimosis

A condition where the foreskin cannot fully retract from the glans when the penis is erect.

If the penis in question simply has a long foreskin with no indication of phimosis, and simply covers the glans even when erect, please consider using long_foreskin instead.

Consider using this tag in times when the foreskin displays a tight opening or "posthion" (where the foreskin bunches up tightly at the tip, extending beyond it, presumably due to its own tension) especially whenever this is the case and the penis is still erect, or the character is very young.