Individual journals can choose to publish manuscripts online as soon as they have been accepted for publication and before they have been copy-edited and proof-read. You may cite an article using its doi at this stage, as long as any citation makes it clear that it is not the final version. Once the articles have been finalised they will be 'published' in the volume to which they belong, and will disappear from the 'Accepted Manuscripts' list.
If a journal is publishing 'Accepted Manuscripts' they will be displayed on the journal's homepage.
If you or your institution subscribes to the journal, you will be able to access the full text of these articles. If you don't have access to the full text, individual articles can be bought online by adding them to your shopping basket.