Verb (5-dan, む, transitive)
1. to fill in (field, entry, etc.); to fill out (form)
休憩中とか、ノートに書き込んでたりしてた。During breaks and such, I would sometimes be writing things in my notebook.
2. to post a message (e.g. on a bulletin-board)
いろんな人が書き込める。Various people can post messages.
3. to store
4. to write (data); to flash