Tray icon is the shortcut for the Avast GUI (Program Window) that loads much later than the actual Avast drivers which protect your system.
Example: Right-clicking the tray icon will offer many choices (program options) to the GUI, or, double-clicking it will bring up the actual Avast program window to your desktop. You interact with this window...Obviously you can't do this early in the boot process, so it loads much later.