What are ‘academic sources’??

Definitions of what constitutes an ‘academic source’ will vary but essentially an academic source represents a scholarly writing that is reviewed by peers.
A publication is considered to be scholarly if it is authored by academics for a target audience that is mainly academic, the printed format isn't usually a glossy magazine, and it is published by a recognized society with academic goals and missions.
A publication is considered to be peer reviewed if its articles go through an official editorial process that involves review and approval by the author's peers (people who are experts in the same subject area.) Most (but not all) scholarly publications are peer reviewed. Some trade publications are actually peer reviewed, but some databases, such as ProQuest, do not consider them when filtering on peer reviewed. This is because getting results from trade publications instead of academic journals can be frustrating to researchers. Instead, these databases exclude these peer reviewed trade publications and only consider publications that are scholarly in terms of content, intent, and audience.