The eIDAS initiative will, over time, make it possible to access electronic services in other EEA countries using the national digital identity held by a person or a business in the EEA country of residence or establishment, respectively. For instance, to access German services using Danish NemID — or Danish services using electronic identities issued by Germany.
Technically, eIDAS is implemented by each participating country operating a gateway service that exchanges the digital identities between the countries whenever users attempt to access services across the inner borders.
WAYF's eIDAS integration aims to provide national electronic identities from foreign EEA countries to web services participating in the federation. One obvious use case is citizens from abroad being able to access Danish student administration systems using the digital identities issued to them by their home countries.
More on eIDAS here.