Brushpacks are pretty much as they sound, packs of brushes XD Someone basically took the time to make some brushes that had some effect they wanted and then kindly made them available to us plebs that either don't know how to make brushes or are too lazy.

This one is one of the ones I've been using for ages. Also still use this one even though most of the brushes in that pack are now actually Krita default brushes, as some of the brush settings are different and I like the original settings for various reasons.

And this one for more foliage (there's quite a few in the modular brushpack I linked to first, this one has some different shaped leaves and you can do more convincing fantasty/anime swooshes with them).

There's probably a tonne more out there but I haven't gotten to a stage where I've felt the need to look for more XD