The file system source is used in order to instruct Curiosity to recursively browse a file system path in order to gather files' metadata. It is particularly usefuly if you have a shared folder where some people add files: you can be notified when an addition happens.
<curiosity>
<sources>
<fileSource
name="aFileSource">
<path>c:\aPath\...\toAFolder</path>
<path>\\anotherPath\...\toASharedFolder</path>
...
</fileSource>
</sources>
</curiosity>
The extracted metadata (which, for the moment, cannot be modified) are: