You can maximize a window to take up all of the space on your desktop and unmaximize a window to restore it to its normal size. You can also maximize windows vertically along the left and right sides of the screen, so you can easily look at two windows at once. See Tile windows for details.
To maximize a window, grab the titlebar and drag it to the top of the screen, or just double-click the titlebar. To maximize a window using the keyboard, hold down the Super key and press ↑, or press Alt+F10.
To restore a window to its unmaximized size, drag it away from the edges of the screen. If the window is fully maximized, you can double-click the titlebar to restore it. You can also use the same keyboard shortcuts you used to maximize the window.
https://help.ubuntu.com/stable/ubuntu-help/shell-windows-maximize.html.en