Data access statements, also known as data availability statements, are used in publications to describe where data directly supporting the publication can be found and under what conditions they can be accessed.
Data access statements are required for all publications arising from publicly-funded research. They are a requirement of many funders' data policies and are a requirement of the RCUK Policy on Open Access (download the pdf, Section 3.3 ii). Inclusion of a data access statement is recommended for publications reporting other research.
Some funders have indicated that they now check for the inclusion of data access statements in publications that acknowledge their support. In particular, the requirement applies to all papers that acknowledge EPSRC funding with a publication date after 1 May 2015.
The aim of the data access statement is discoverability - the data referenced by the statement do not have to be openly available. There are many reasons why access to data should be restricted and if you are unsure about whether you should publish your data openly please firstname.lastname@example.org for advice.