Save important information for later
To quickly jump back to important messages or chats, you can mark, pin, or save them. That way, you'll always have them at hand.
Mark a message as important
You can mark a message as important for all chat members. To do this, press it and select Mark as important. To open the list of such messages, press three dots → Chat info → Important messages.

Pin a message
To keep track of important updates and decisions, select such a message in the chat and press Pin message. After that, the message is pinned at the top of the chat or channel for all its members. Pressing a pinned message takes you back to it and highlights it in the chat.
You can only pin one message per chat.

Save a message
To save an important message for later, press it and select Add to Saved messages. After that, the Saved messages chat will appear where you can view that message at any time.

To save a file from a chat on your device, download it. How to do this
Mark a chat as unread
To make sure you don't forget to reply to a message, mark the chat with it as unread and return to it later. To do this, select a chat and press Unread.
The chat will appear as unread only to you. Other chat participants will see the messages as read.