A state is a temporary psychological or emotional condition, such as feeling anxious or energized, that fluctuates within a person from moment to moment.
States are what traits are not: brief, situation-driven, and changeable. Feeling nervous before a job interview is a state — it will pass once the interview is over. Being a generally anxious person, someone who tends to feel that way across many situations and over long stretches of time, is closer to a trait. The same word, "anxious," can describe either one depending on the timescale.
This distinction matters for how you interpret a self-report question. If someone answers "I feel anxious" based on how they feel today, that's a state-level answer. Good personality assessments are deliberately worded to pull for the trait-level pattern instead — asking about general tendencies ("I often feel anxious in new situations") rather than momentary experience, and typically framing questions around how someone behaves across time, not in the current moment.
Traits and states aren't fully separate, either. Someone high in trait Neuroticism will experience negative-affect states more frequently and more intensely than someone low in it, even though the state itself still comes and goes.