Nuclei form the center of crystal growth during solidification of alloys.

Every crystal starts growing at a nucleus and continues its growth until it forms contact with its neighboring crystals. In micrographs, these margins become visible as grain boundaries. A high nucleus count in the solidifying melt implies a large number of crystals and thus a fine-grain structure. In contrast to that, coarse-grain structure is obtained with a deficiency in nuclei.

Nucleation is referred to as dynamic if nucleation or the solidification sequence is influenced by vibration or oscillation.

Additional references:
Nucleation conditions
Balance of nuclei