What exactly is Gradle in the Android studio?

Gradle is a build system.

Gradle absorbed both Ant's freedom and Maven's merits of convention. And the issue of XML, the biggest drawback of both Ant and Maven build tools, is also being addressed using Groovy language.

As a project configuration/build tool, project configuration is static configuration information and build is dynamic. However, by allowing this to be described in XML suitable for storing static data, it imposes significant restrictions on the building, which is a dynamic act. In addition, XML is so redundant that it has more XML frameworks than actual settings.

Gradle is written in Groovy DSL, and configuration information is in the form of putting values in variables, and dynamic builds can be made by calling the plug-in for Gradle with Groovy scripts or by coding directly. References Reference 2