What does it mean when something is said to be predictable?

That there is an effective model that captures the behavior.

Predictable is not some objective free-floating fact.

Predictable to whom?

There are some systems that are predictable to some participants but not to others.

When you have relevant knowhow the system becomes more predictable in a way that is difficult to explain to others.

An effective predictive model is one that can produce accurate predictions… and do so cheaply and quickly enough to actually be useful in practice.

A participant who has an effective predictive model in a context has a significant advantage over entities who don't.

If no one has an effective predictive model in a context, the entity with the fastest OODA loop will win.