The taskbar is shown by default on Remix OS. To hide the taskbar, position your mouse pointer on the taskbar and right-click on your mouse. The 'Hide taskbar' button ( ) will appear above the taskbar. Clicking on it will hide the taskbar.
If you'd like to show the taskbar after it's been hidden, position your mouse pointer over the area where the taskbar would normally be. The 'Show taskbar' button ( ) will appear where your mouse pointer is. Click on it to show the taskbar.