According to Francis Bacon, the four idols are the four main fallacies or falsehoods that prevent people from gaining true knowledge and becoming the best versions of themselves. They consequently stop civilizational progress. These four idols are those of the Tribe, the Cave, the Marketplace, and the Theater.
Idols of the Tribe
The Idols of the Tribe refer to the entirety of the human race—they're founded "in human nature itself, and in the tribe or race of men." Bacon argues that the human senses are actually unreliable and can often fool and deceive people, as we all tend to see what we want to see and we're prone to assumptions and generalizations. What we see is actually a "false mirror" of reality.
Idols of the Cave
The Idols of the Cave refer to the flaws of people as individuals. The thoughts that emerge from the depths of our mind, symbolically presented as a cave, are influenced by our environment and by our personal interests and beliefs—so much so that we fail to see the "light" or the true meaning of the world around us and the things in it.
Idols of the Marketplace
The Idols of the Marketplace refer to society, or the errors that arise from the "intercourse and association of men with each other." People use words to voice out their thoughts and opinions but often do not particularly care to be sensible or rational and end up choosing "ill and unfit" words, which often lead to mass confusion, vulgarity, and controversy.
Idols of the Theater
The Idols of the Theater refer to the errors in education, knowledge, philosophy, science, and religion that people accept as universal truths without any proof. These are the dogmas that fool both individuals and societies and force the people to be narrowminded and unwise, which in turn stagnates social, political, cultural, and scientific growth and development. Bacon calls them "Idols of the Theater" because in his judgment, "all the received systems are but so many stage plays, representing worlds of their own creation after an unreal and scenic fashion."