Chat apps offer various forms of encryption. Some are more secure that others. For example, Signal and WhatsApp offer end-to-end encryption, which prevents anyone else but the person you are talking to from reading your messages. Facebook Messenger, WeChat, Tencent QQ and Telegram do not use end-to-end encryption by default.

End-to-end encryption protects messages in transit all the way from sender to receiver. It ensures that information is turned into a secret message by its original sender (the first “end”) and decoded only by its final recipient (the second “end”). No one, including the app you are using, can “listen in” and eavesdrop on your activity.

Accessing end-to-end encrypted messages in an app on your device actually means that the app company itself can’t read them. This is a core characteristic of good encryption: even the people who design and deploy it cannot themselves break it.

Electronic Frontier Foundation (11-24-2018). Retrieved from:
https://ssd.eff.org/en/module/what-should-i-know-about-encryption

No technology is 100% secure. Even end-to-end encryption does not guarantee that your conversations might be vulnerable for outsiders. Read for example this article by Gregorio Zanon about why end-to-end encryption does not prevent Facebook from accessing WhatsApp messages.