Telegram the messaging app employs end-to-end encryption so that no intermediate parties (like Telegram the company or your ISP) knows about the content of any chat messages. Only the message sender and recipient knows the content of the message.

Telegram is open source. Their source code can be audited for a correct implementation of the aforementioned end-to-end encryption, and compared against official build of the app.

Note that the following scenarios may not match your definition of secure depending on your threat model. None of them are inherent to Telegram and no messaging apps would have realistic protections against such attacks:

- Message is encrypted only between the two ends. It is not encrypted on your phone nor the other side's. Any malware installed on either devices would expose your messages to attackers. Likewise, unofficial Telegram builds may carry security-compromising components not present in the official release.

- If the person you are messaging with is malicious or being impersonated, you may reveal confidential information to third parties through Telegram chat.

- Telegram accounts are tied to phone numbers. If you lose your phone number or other people can intercept your SMS, attackers may be able to hijack your Telegram account. Two factor authentication provided by Telegram help mitigate this risk.

- You may be "persuaded" through force or threat to allow attackers have access to your phone.

- When facing state actors, the fact that you are using Telegram for any reason could be ground for further investigation.
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