For me, it's just about how the core of a team is configured.
One star - some teams like LeBron's Cavs or Hakeem's Rockets (pre-Drexler) built title contenders around only one legitimate star. A one-man show. MVP candidate.
Two stars - Kobe and Shaq, a dynamic duo. Usually a pair of MVP or at least top ten talents together (to be a title contender).
Three stars - the 2008 Celtics, a Big Three. Three perennial All-Stars where at least 1-2 are All-NBA guys (to be a title contender).
Four stars - Usually one high level star (All-NBA or MVP guy) on a max contract with three near to borderline All-Stars usually on $10-12 million apiece (to be a title contender). The current Bulls team is a lot like this with Derrick Rose and Deng, Boozer, Noah (well, planned to be that, Boozer has fallen off).
More balanced -- say 2004 Pistons. That type of squad. They had four/five high caliber starers and probably the best bench in the league. Billups and Big Ben were top five players at their respective positions. Sheed and Rip were top ten arguably top five guys at their positions. Tayshaun Prince was an above average starter. Lots of top tier talent needed (to be a title contender). No MVP guys or guys who are top ten players in the league.
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So, to repeat, for me it's solely about how a team's core is configured.
Just a common and easy short-hand way to refer to a team that is built around three stars.