See Notifications and the Notification Area. It says:
The notification area has been known historically as the system tray or status area.
The proper name is Notification Area. That article describes how a program can use the Notification Area. Note that the program must put itself there, it cannot be put there by some other program. Be sure to read the Guidelines in that article. I do not know what the "Telegram" program is but developers have often not followed those guidelines for the use of the Notification Area.