This message is shown when you select one or more files in the Commit dialog, right-click the selected file(s) and choose 
Revert from the context menu.
The built-in Commit dialog has two panels: Unstaged files and Staged files.
Reverting files listed in the Unstaged files panel restores their Index state, that is, the latest staged for the commit state of that file. If the file has not been staged recently, then it is reverted to the latest committed state of that file.
Reverting files listed in the Staged files panel restores their HEAD state. That is, the latest committed state of that file.
To discard changes in the selected file(s), click Yes. To cancel the operation, click No.