Predatory publishers are those that, under the umbrella of open access, have a dubious reputation due to the fact they seem more interested in charging APCs than publishing with quality criteria. Indeed, it is often the case that they do not offer the editorial services typically provided by journal publishers, such as indexing journals in databases or peer review processes to verify the quality of articles. They sometimes contact potential authors directly, e.g. targeting doctoral students and offering to publish their theses.
We recommend avoiding these publishers, as linking your name or work to them can be counterproductive.
You can query the Directory of Open Access Journals (DOAJ) to check if a publisher is considered a "predator"; this portal contains over 12,000 open access journals that are considered to be of high quality as they meet the established criteria, all of them have a peer review process, and many are indexed in databases.
The Think, Check Submit portal has a Checklist to help you assess journal quality, allowing you to decide which ones to publish with and which ones to avoid.