Read receipts let your contacts know when you've seen their message. They are optional, and turned off by default. Here's how to enable them:
Mobile (iOS and Android)
1. Open Settings by tapping your profile picture in the top left of the main screen.
2. Tap "Privacy" to open your privacy settings.
3. Enable "Read Receipts".
Desktop
1. Open Settings by clicking the cog (⚙️) icon
2. Click "Privacy" to open your privacy settings.
3. Enable "Read Receipts".