What is meant by nucleated in this context?
Cytoskeletal nucleation refers to the de novo formation of cytoskeleton filaments. If microtubules are nucleating, that means microtubules are being polymerized from lit. "From the beginning" by α/ß-tubulin dimers. So literally, and we see in the image below, Arp2/3 complexes act as progenitor sites so to say for new actin filaments by the "nucleation" of actin:



From wikipedia for Nucleation: "Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organisation."