When you save a file, the mrclean command will be executed. It's very usefull if you are bored of the .gch files made by GCC with the linter-gcc package.
If the box is checked, it will use your mrclean executable instead of the default one.
The default executable delete files matching all the following regex:
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.