These are newly published articles within a journal but not yet assigned to an issue. The Online First service lets users access peer reviewed articles well before print publication. These articles are searchable and citable by their DOI (Digital Object Identifier). With Online First publication the time it takes for critical discoveries to reach the research community is significantly reduced.

Users will be able to access online first articles via the "All Volumes and Issues" pages for the selected journal.

See also:

Where can I find Online First articles for a journal?

Why has an article not been indexed by Web of Science?