Idiom "be supposed to"

This idiom seems to be used often. I often hear it in English conversation. But I couldn't understand its meaning enough even if I checked a dictionary.

Thanks to Murphy book, finally I somewhat started to grasp the meaning. So when I heard the idiom in a drama the other day, I was really happy to be able to understand it! But it will take me some time to be able to use it in my conversation.

If I want to put an article on my blog in the daytime of weekday, I have to do it by sending the article through my cell phone, because we are not supposed to use the Internet for private purposes in most office.