The fundamental postulate is a concept developed by George Kelly, who is considered the first cognitive theorist. Kelly theorized we all have 'personal constructs' which are individual differences in how we perceive, interpret, and predict situations in our environment and the behaviors of other people.
These differences affect how we interact with our environment. Our personalities are essentially the way we interact with the world. The fundamental postulate is that we act in a way with how we expect the situation to be based on past events. How we interpret and interact with the world predicts how we behave in future scenarios and influences how we behave.
When you go to the grocery store you get your items, wait in line, pay, and leave the store. You do this based on previous encounters and interactions at the grocery store in the past. If an individual perceives people as being nice and trustworthy based on their interactions and perceptions with people then they are more likely to seek social settings and be trusting. If an individual perceives people to be mean and dishonest they are more likely to be antisocial and distrusting of others.