A State is in effect an on/off switch. When the State is active, the model is in that state; when the State is inactive, the model is not in that state.
States are primarily useful when carrying out Agent Based Modeling. One or more sets of States may be placed in an agent to describe the current state of that agent. For instance, if you are modeling a population of individuals, you could have your agent definition contain a State representing whether or not an agent was a smoker. Similarly you could have a State representing whether or not an agent was a female. For continuous variable (such as age) a Stock should be used; but for binary or dichotomous variables, a State is a more natural primitive.
A State is configured using its initial value equation. This equation may be a simple value such as: 1 or 'true' for active, or 0 or 'false' for inactive. The equation can also be a logical equation such as:
not true # Which is false
Or a simple mathematical equation:
5*5 > 10 # Which is true
Or a function that depends on other primitives:
not ([Smoker] and [Diabetic])