It does what it says, it purge the central cache. You can also manual delete it from your system, the path is shown in Bridge preferences Cache section.
When you restart Bridge it creates a new but empty cache meaning it has to build your thumbs and previews again. No more, no less. This can take some time depending on amount of files, file size and the power of your system.
If you want all files cached again the easiest way is to quit as many apps as possible and in Bridge select the main folder holding your files and subfolders.
Use menu view / show items from subfolders. This will cache all the files in your main folder automatically. But as said, it can take a bit of time, I often let it run overnight and have a fresh start in the morning