When I edit files in my ~/.vim, the .netrwhist file would mysteriously be changed, too.
It's content:
let g:netrw_dirhistmax =10
let g:netrw_dirhist_cnt =6
...and so on...
What does this file do? Is it important?
netrw is a kind of vim plugin/script which supports reading and writing files across networks. .netrwhist is a history file which maintains all the directories that were modified. So whenever you modify the contents of ~/.vim it adds one entry in .netrwhist
A sample .netrwhist is as shown
let g:netrw_dirhistmax =10
let g:netrw_dirhist_cnt =6
let g:netrw_dirhist_1='/Users/wolever/EnSi/repos/web/env/web/lib/python2.6/site-packages/django'
let g:netrw_dirhist_2='/private/tmp/b/.hg/attic'
let g:netrw_dirhist_3='/Users/wolever/code/sandbox/pydhcplib-0.6.2/pydhcplib'
let g:netrw_dirhist_4='/Users/wolever/EnSi/repos/common/env/common/bin'
let g:netrw_dirhist_5='/Users/wolever/EnSi/repos/common/explode'
let g:netrw_dirhist_6='/Users/wolever/Sites/massuni-wiki/conf'
netrw_dirhistmax indicates the maximum number of modified directories it stores in the history file. ie Max History Size. netrw_dirhist_cnt indicates the current history count of modified directories.
If you want to disable netrw to generate history file, then see this.