Spotlight, the intelligent search function of OS X, is very fast because it creates and uses an Index of all files on all connected hard disks. This option rebuilds the index. A few seconds after deleting it, the system will automatically rerun the content indexing of the hard disks in the background. You can see the progress of this action by clicking on the Spotlight icon in the menu bar. When you restart your computer, the indexing is stopped and will be resumed after a restart.
If Indexing is turned off, removing the index will automatically reactivate it.
The Spotlight index can be rebuilt from the Rebuild command in the Maintenance pane or from the Automation pane.