Feathering is the broadening of a line by capillary action along the fibers of the paper. In other words, it doesn't just broaden the line, it makes it look like an ugly caterpillar. Feathery paper also often has the disgusting habit of printing through to the other side of the page where it leaves random blots here and there. Yuck.
Here are a few close ups of various inks on Levenger Circa paper. The x-height of the letters is around 2mm:
The following is an example of a paper that does NOT feather. It's from a Kunst & Papier journal which has fountain pen friendly paper. Although the paper isn't super smooth like Clairefontaine, you can see in the thins created by this cursive italic that the ink isn't spreading out or getting catepillary:
Anyway, hope this helps.
Doug