Local watchdog Roskomnadzor have also stated that they have begun to block access to Telegram following the decision by the Russian court. For those unfamiliar with the Russia-Telegram drama, basically Telegram had refused the request of the Russian government to hand over encryption keys.
Apps such as Telegram and WhatsApp boast end-to-end encryption, meaning that messages are encrypted when they are sent and decrypted upon arriving to their destination with the correct key. This means that even if hackers were to intercept the message in transit, as long as they do not hold the decryption key, they will not be able to read it.
This is what the Russian government had requested of Telegram, where the government wanted the keys which they claim was for security purposes, where it would allow them to decrypt messages sent by criminals and/or terrorists. Similar requests have been made to other companies in the past, such as Apple where the FBI had requested the company to build backdoors into their iPhones.