The App Center Test task is used to run your mobile app test suites in App Center Test. App Center Test provides a test automation service for native and hybrid mobile apps. Tests written in supported frameworks can be run with little modification on hundreds of unique device model and operating system configurations hosted in a Microsoft data center.
App Center Test is a service of Visual Studio App Center. Visual Studio App Center brings together multiple services commonly used by mobile developers into an integrated cloud solution. Developers use App Center to Build, Test, and Distribute applications.
Documentation for using this task is at Using Azure DevOps for UI Testing.