Complex sentence: Any theory "T" categorical in some infinite cardinal κ is very close to being complete: all infinite models are equivalent to some model of cardinal α by the Löwenheim-Skolem theorem, and so are all equivalent as the theory is categorical in κ.
Split into simpler sentences: Any theory "T" categorical in some infinite cardinal κ is very close to being complete. The reason is that all infinite models are equivalent to some model of cardinal α by the Löwenheim-Skolem theorem, and so are all equivalent as the theory is categorical in κ.