"Publication fees charged by certain
open access journals post-acceptance, are also known as author
publishing charges or article processing charges (APCs). APCs may be driven down
by submission fees, particularly among open access journals with high
rejection rates. In contrast to post-acceptance charges by traditional
journals, these APCs are more often flat fees because they primarily
fund peer review and online dissemination, which are length independent.
Be aware that “predatory” journals may take advantage of the
APC-based model to receive payment in return for minimal peer review and
processing, so be sure to look for warning signs and consider checking whether your target journal is listed by the Directory of Open Access Journals. A truly open access journal should also meet the two-fold requirement defined above by PLOS:
“unrestricted access and unrestricted reuse,” meaning that an open
access article must not only be freely accessible to readers but also
freely available for copying, distribution, and derivative work, as long
as the original author is acknowledged. In particular, open access
articles are often associated with a CC-BYlicense, although certain journals may not support reuse/derivation."