Just ⌘ Command-Drag & Drop it away from the menu bar, this way you can also re-arrange your icons. This trick will only work for OSX icons (e.g. Time Machine or WiFi), for non OSX native icons (e.g. Dropbox or GeekTool) you need a third party app like Bartender or Menu Bar Rearranger.
Best Answer
Looking at the position, it seems to be an app from Apple.
Press Command and drag & drop it out of the menu bar.