What is os.log for?

I am going through a few Swift tutorials on how to build simple apps, as I am just starting to code. I want to make my app iOS 9 compatible, as I have an iPad 3. However, all the os.log statements generate an error in Xcode which tells me to add an if #avaliable statement before any of the os.log statements. What does os.log do, and if I need it, is there an issue using an if #avaliable statement for iOS 9 compatibility? If not, what is the equivalent code for iOS 9 to go in the else statement after the if #avaliable statement? Thanks.