An Article Processing Charge (APC) is a publishing fee charged to authors who are publishing in open access journal. The fee shifts the journal production cost to the author and replaces the subscription charge that libraries and researcher would pay to access an article behind a paywall.
The method for computing acceptance rates may vary among journals.
Typically it refers to the number of manuscripts accepted for publication relative to the number of manuscripts submitted within the last year.
More online and print sources from various fields can be found at these library FAQ pages:
St. John's University: Journal Rankings and Acceptance Rate
American Psychological Association (APA) provides historical data on their affiliated journals and the data can found at the Journal Statistics and Operations Data.