Blazor.EventAggregator is a lightweight Event Aggregator for Razor Components. Razor Components (and formerly known as Blazor) is an upcoming technology included in ASP.NET Core 3.0 (currently in Preview 2).
Event aggregators are used for indirect component to component communication. In event aggregator patterns, you have message/event publishers and subscribers. In the case of Razor Components, the component can publish its events and other component(s) can react to those events.