Build runner is a part of TeamCity that allows integration with a specific build tool (Ant, MSBuild, Command Line, and so on). In a build configuration, a build runner defines how to run a build and report its results.
Technically, build runners are implemented as plugins. Each runner has two parts:
the server-side settings that are configured through the web UI
the agent-side part that executes a build on an agent
Instructions on configuring bundled runners are listed under the Configuring Build Steps section.