Telegram is a messenger focused on privacy. All your conversations are encrypted, which is disliked by some countries including Russia.
Telegram was asked to give the decryption key to the Russian government, but they refused and were fined 14000$
President Putin approved a new law in 2016 saying that all communication platforms have to work with the government and let it decrypt everything that users share or talk about.
If Telegram refuses to reveal their decryption keys for longer, they will get another fine and will soon be completely blocked in Russia. According to stats from late 2017, there are approx. 9.5M users of Telegram in Russia just because it's private and safe to use.