تَقْوِيَةُ الْحُكْمِ إِذَا كَانَ الْخَبَرُ فِعْلًا
Strengthening a judgment when its predicate is verbal.
الْهِلَالُ ظَهَرَ
The new moon—it has appeared.
Al-hilal is the initial subject, and the implicit subject of zahara refers back to it. The moon is first established as the topic and then predicated of through a verbal clause.
The construction allows repeated predicative linkage: an independent topic is named, then a verbal judgment is attached to it. This can make the affirmation more prominent to the listener.