You know how when you meet someone new, you make guesses about what kind of person they are based on how they act and what they say? That's called making inferences about their personality.
But sometimes, even when someone doesn't say a lot or do anything particularly special, we still make assumptions about them. That's called spontaneous trait inference.
It happens because our brains are really good at picking up on patterns, even when we're not really paying attention. So if someone seems quiet and doesn't say much, we might assume that they're shy. Or if someone is really outgoing and friendly, we might assume that they're nice people.
It's important to remember, though, that these assumptions aren't always accurate. Just because someone seems quiet doesn't mean they're necessarily shy, and just because someone is outgoing doesn't mean they're always nice. It's always best to get to know someone before making any major judgments about their personality!