Article processing charges (APCs), were created as one way for a publisher to meet author or funder demand for open access and at the same time generate the income required to cover publishing costs (and maintain or increase profit margins). Mostly these fees are paid by universities or funding agencies via grants. Charges can be $500-$4,000, or more.
Some publishers may waive the fee if asked for those in developing economies - no harm in asking. Also, some scholars may not be associated with a university or may be early career without resources for covering these charges.
OA advocates generally object to hybrid journals where only the articles whose authors have paid their APCs are open, (instead of the whole journal) and the rest of the articles are then toll, that is, by subscription. This practice is called "double-dipping" as institutions may land up paying for the content more than once. APCs are determined after an article is accepted for publication/post-peer-review, and should not be confused with submission fees.
APCs were meant to level the playing field particularly for scholars in disciplines where grant funding is unavailable, or for junior faculty without grants. It is questionable if shifting the burden to authors or their universities to pay for open access publishing is a sustainable model, or in the spirit of equitable access and worldwide dissemination of research results.