Any modification of the source code which you introduce. If a change has been committed to the version control system, but not yet included in a build, it is considered pending for a certain build configuration.
TeamCity suggests several ways to view changes:
The Changes page shows the list of changes made by TeamCity users and how they have affected different builds. By default, your changes are displayed. The page has a users selector enabling you to view changes made by any other TeamCity user the same way you see your own changes.
Pending changes are accessible from the Projects page, build configuration page, or the build results page.
Viewing and analyzing changes involves the following possibilities:
Observing actual changes that are already included in the build using the Changes link on the Projects page or on the Changes tab of the build results.
Observing pending changes in the Pending Changes tab of the Build Configuration Home Page.
Viewing the revision of the sources corresponding to each change
Ability to modify the change comment in TeamCity available to project administrators by default (changing the comment in the VCS as well is recommended for consistency)
Navigating to the related issues in a bug tracking system.
Navigating to the source code and viewing differences.
Starting an investigation of a failed build, if your changes have caused a build failure.
User's Guide: Investigating and Muting Build Problems