Publishers using the gold open access model (where the journal itself is not funded by an institution through subscriptions, but rather by the authors who publish their works in that journal) to receive payments from authors while neglecting to undergo a true peer-review process.
"These [predatory publishers] are OA [Open Access] journals that exist for the sole purpose of profit, not the dissemination of high-quality research findings and furtherance of knowledge. These predators generate profits by charging author fees, also known as article processing charges (APCs), that far exceed the cost of running their low-quality, fly-by-night operations." - Berger and Cirasella, page 132.
from Times Higher Education: https://www.timeshighereducation.com/news/infographic-journals-and-publishers-setting-sights-on-the-unwary
Image taken from Stony Brook University Libraries, Scholarly Communication: http://library.stonybrook.edu/scholarly-communication/know-journal-legitimate/