These are citations submitted by publishers to PubMed for articles that appear on the publisher's web site in advance of the journal release. Citations listed in PubMed as Online ahead of print may not be available at your library until the print version of the journal is published. Many publishers limit online access to "ahead of print" articles to individual subscribers; charging others for obtaining the "ahead of print" article, sometimes also including libraries with paid subscriptions.
If a "Free" link to the full-text article is provided, you should follow the link to obtain the article.
If a publisher provided link takes you to a fee-based service, you could decide whether to purchase the article directly from the publisher. You may want to discuss the availability of the article with your library.