With the release of Apple’s Yosemite last year, Apple announced that older versions of the Mac software (named “OS X”) would no longer be supported. This means if you’re using an old version then you won’t receive any security fixes to prevent the prolific hacking present on the internet today.
So if you bought your Mac computer prior to around October or November 2014, and haven't upgraded the software to OSX Yosemite, then your computer could be at risk!
The good news is that for most computers it's free to upgrade to the latest version, which also enables you to take advantage of Apple's latest features.
To find out more, visit https://www.apple.com/osx/how-to-upgrade/