Article processing charges are associated with the articles that are accepted and queued for typesetting and author proofing ; The charges are required to pay before processing your article for pagination.
The cost is to cover the article production for the following
In-house copyediting and linguistic support
Pagination & typesetting
Inclusion in Crossref, giving a permanent DOI and enabling easy citation in other journals.
Archiving, where appropriate, in an internationally recognized, open access repositories.