وَإِنْ حَلَقَ رُبُعَ رَأْسِهِ فَصَاعِدًا فَعَلَيْهِ دَمٌ
Translation: If he shaves one-quarter of his head or more, an animal offering is due.
Rubʿ means one-quarter, or 25% of the head. Fa-saʿidan extends the ruling to amounts above that threshold.
This concerns impermissible shaving during ihram. The prescribed shaving/trimming at the proper stage of Hajj or Umrah is a different, legitimate rite and must not be confused with this infringement.