An APC is a publisher’s fee for making a paper freely available online. The cost of publication is moved from the reader (via subscriptions) to the author or institution (via an APC).
This is often referred to as Gold Open Access, although there are Gold Open Access journals which do not require an APC. Some publishers will make already published papers Gold, by charging a 'retrospective APC'.
There are a growing number of Gold Open Access journals to publish in; why not look up how many there are in your field by searching the Directory of Open Access Journals