For the first ever in the Mac operating system, you can hide the menu bar in Mac OS X El Capitan. I’m not sure why you would want to, but if you do, here’s how:
Open System Preferences.
Click on the General icon.
At the top of the new window you’ll have several options under the Appearance listing.
In addition to enabling or disabling the dark menu bar and Dock, you can choose to "Automatically hide and show the menu bar." Click this box and the menu bar vanishes. It will show up again when you glide your mouse arrow towards the top of the screen.
Don’t like it? To make the menu bar stay visible at all times, recheck the "Automatically hide and show the menu bar."
OS X El Capitan is available as a free update from the Mac App Store. The operating system upgrade supports all Macs introduced in 2009 and later, and some models introduced in 2007 and 2008.