Applications are available as part of the Enterprise Edition.
An Application allows you to aggregate multiple projects into a kind of meta-project. For instance, a set of projects which has been split for technical reasons, but which shares a lifecycle can be treated as a single entity in SonarQube with a unified Project Homepage, Issues list, Measures space, and most importantly: Quality Gate.
Applications are created and edited in the global Portfolio administration interface: Administration > Configuration > Portfolios. For more, see Configuring Portfolios and Applications.