Prior knowledge is the knowledge the learner already has before they meet new information. A learner's understanding of a text can be improved by activating their prior knowledge before dealing with the text, and developing this habit is good learner training for them.

Example
A group of young learners are going to read about dolphins. First they talk about what they already know in a brainstorm activity.

In the classroom
Pre-task activities are a good way to explore and share prior knowledge. Making predictions about content, answering true or false questions, agree on ‘5 things you know about...' and class or group brainstorming are all effective tools.