Extraneous load is when cognitive load is imposed on the learner, because the teacher is trying to teach several unrelated concepts in a way that increases their cognitive load. This is bad cognitive load because it’s avoidable. Consider having a slide with three unrelated concepts all next to Esch other. Or worse, they’re somewhat related but it’s unclear (to the learner) how they’re related. The learner is spending mental energy (specifically using up working memory) trying to sort it out instead of learning one of the concepts well enough to commit it to long term memory.