How to use GateKeeper on your Mac

[iMore.com]

GateKeeper is one of the Mac's main lines of defense against malware.
GateKeeper has been built into OS X since the days of Mountain Lion, but if you open up Spotlight and search for it, you won't find an app or system preference by that name. GateKeeper is what Apple calls OS X's defense against "malware" ? malicious software. GateKeeper restricts how software from third-party developers runs on your Mac, so understanding what it is and how it works can be pretty important.

Full story from the iMore Blog...

 

[Ran03]

but, my macbook air mid 2012 doesn't have the choice for "everywhere" in the security and privacy. what should i do? please help me.