Here’s a list of some icons and references to understand what they mean:

Sending
Sent
Delivered
Read
Disappearing Messages
Quoted Reply threads for Android
Sealed Sender
Media can be viewed more than one time.
View-once media message
View-once media message, viewed
Select
Set up an iPad as an independent device or unlinked device.
Contact is saved to your phone’s contact list.
Note to Self
Safety number has been marked as verified.