Depending on your workplace, this больничный лист — sick leave certificate — may be the most important document you get or something to ignore. It's an official document issued by a licensed medical provider that certifies that you really were sick last week, not just goofing off, hungover, or painting the garage at the dacha.
When you are too sick to go to work, you have to immediately "open" the certificate by going to your clinic of choice. Then you return when you are better and "close" the certificate. You deliver the certificate to the HR department at work, where they will determine your salary for the days you were out of the office, which depends on how long you've been working there and other factors.
Often you'll be feeling better but either you can't get an appointment to "close" out your certificate, or the doctor thinks that bronchitis demands two full weeks in bed. You can go back to work ahead of time, but you will be paid sick leave rates until the certificate is closed. So you might as well stay home.