A predator is a person or organization that ruthlessly exploits others.
Such journals lie about conducting peer review. They only want your money and will publish almost anything.
Therefore your publication with them has no prestige because it was not REALLY peer-reviewed. Scholars to not trust the accuracy of papers published with such journals.
Yes, Michael is right. Just check if a journal is asking authors to pay a publication fee. It could be worse: some journals make you believe you have nothing to pay. Then, they "accept" to publish your paper...under the condition you pay a fee.
Be careful too when a journal accepts your article very quickly, that is in a few days or in a week ore two. This is obviously impossible if they really do peer-review.
Predatory journals most often lie their indexation. As a rule you need to pay for publishing in them, and they are kot indexed in the basis they claim they are. For example in Croatia you need papers to be published in indexed journals in order to develop your academic career.