Team Foundation Build stores the files necessary to run the build in a folder in version control specified in the Project File section of the Build Definition Dialog. A quick way of navigating to that folder in the Source Control Explorer view is to right click on the build definition and select “View Build Configuration Folder”.
All the files in the root of this folder are downloaded every time the build is performed. The TFSBuild.proj file is the MSBuild project file that controls the overall process performed by the build. If you wish to customize this (for example, to modify the parameters passed to Ant), then you should get the latest version of the file, check it out, make the desired changes and check-in.
For more information on the TFSBuild.proj file customization see MSDN: Customizing Team Foundation Build.