To infer means to predict or guess, and how do we do that, we look at all the available cues in a situation, connect it with our experiences in life or what we already know. This previous knowledge is also called schema. In other words, things we already know. Every new piece of information we receive, we connect it with our previous experience or knowledge to understand how things work.
This helps us to predict what will happen in each situation.
For example, if we see a boy is hurt and there is a banana peel next to him. Looking at the situation we will be able to infer that the boy must have slipped on the banana peel.
Inferencing is a life skill too and essential for effective communication. It helps us to understand another person's perspective and even understand what a person means by his body language, gestures and facial expressions.