The term predatory journal refers to journals that are profit driven and do not intend to further the production of scholarship. Often the publishing entity will request money in exchange for the author to publish with them. In the world of publishing and open access publishing it is not uncommon, or unheard of for a publisher to make publishing a "pay to publish" opportunity. However, the intention of predatory journals is to gain funds from an author, without furthering scholarship or actually producing journals.
Some of the indicators that you might be dealing with a predatory journal include:
Think.Check.Submit is an initiative to help researchers evaluate journals before submission, and identify when journals may be predatory.