When I send a message on Messenger and go back to the 'main page' where I see all my conversation, there will be one of these ticks next to the message:
What is the difference between these two?
The first tick indicates that the message has been delivered to the Facebook server.
The second tick indicates that the message has been delivered to the recipient's phone. It doesn't necessarily mean that the recipient has read it.
If the profile picture of the recipient appears, instead of the tick it means that the recipient has read your message.