Predatory journals, or 'fake journals' take advantage of the open access publishing model, and the current pressure on academics to publish. Disguised as proper Open Access journals they send emails to academics asking for submissions and promise a rapid peer review process. The peer review process is only rapid because it usually does not happen at all in these journals. Predatory journals usually publish everything they receive, which some academics have taken advantage of to expose them, e.g. the famous star wars paper.
If you receive an email from a journal that seems suspicious, there are a few warning signs you can look for:
Poor grammar and incorrect spelling (both on website and in the email)
The speed of peer review is too quick to be true
Editorial board contains fake members, or members who don't know they are on the board
Scope of journal is too broad
Journal boasts about being on Google Scholar
No one in your field has ever heard about the journal before
You can also use the Think. Check. Submit. resource or the DOAJ as a guide to find a safe Open Access journal.