The system tray is located in the Windows Taskbar, usually at the bottom right corner of the screen next to the clock. It contains miniature icons for easy access to various system functions such as antivirus settings, printer, network, sound volume, battery status, and more. Hover over or click (or right-click) an icon to view and access details and controls for that icon.
The example below contains the stopwatch icon for RestBreak plus various other icons that you might find on almost any PC.
The system tray is loaded by SysTray.exe at system start-up and may load various icons depending on how your system is configured. Your system may or may not have the icons shown in the example above.
Clicking on the triangle or up-arrow on the left side of the system tray displays any hidden icons and offers the option to configure which icons are hidden or not.
In Windows 10, go to Settings / System / Notifications.