Who can explain for what are these "subscrided threads" created? What do they do?

the forum emails you everytime someone posts in a thread your subscribed to.

i believe you have a choice of a single email daily containing every update for said day, weekly update and monthly update.

Everytime you post in a thread, you subscribe to that thread. Onlythreads you subscribed will show up in your user CP, and if you set it to send you an email whenever someone replies, will send you an email.