Filters let you postprocess your output. Think of it like instagram for websites, but useful. You can configure per file extension a series of python functions or shell commands, which will be applied in place to the output file.
For example, suppose you want to apply yui-compressor to your CSS and JS files:
There, filters.yui_compressor is a simple wrapper around the command so that it applies in-place to the output files.
If you use strings there (untested), they are taken as commands. The "%s" will be replaced by the filename, the usual crazy shell quoting rules apply:
Keep in mind that the filters modify the output of Nikola, not the input, so your images, CSS, and JS files will not be touched in any way. And of course changing the filters applied to a file will force a rebuild, so you can experiment freely.
Having many separate CSS or JS files is usually a nono for performance reasons because each one may involve a separate HTTP transaction. The solution is to "bundle" those files in a single, larger file.
The reason not to do that is that usually it means having a huge, uncomfortable thing to handle. So Nikola tries to give you the best of both worlds, by letting you have separate files, and bundling them (or not) on build.
There is a new option, USE_BUNDLES that defaults to False, and there are some changes in the theme templates so that it uses the bundled version when needed.
This was only possible thanks to Webassets. However, if you don't have Webassets installed, or you don't enable USE_BUNDLES, this should cause no changes in the output.
These new features will allow Nikola users to improve their site's performance with minimal tweaking, which is always a good thing.