Static variables retain their assigned values over repeated calls to the function. They’re basically like global values that are only “visible” to that function.
The initializer statement is only executed once however.
This code initializes dateFormatter to nil the first time the function is used. On every subsequent call to the function a check is made against value of dateFormatter. If it’s not set (which will only be true the first time) a a new dateFormatter is created. If it is set then the static dateFormatter variable will be used instead.
It’s worth becoming familiar with static variables. They can be very convenient but have downsides too (in this example it’s impossible to release the dateFormatter object for example).
Just a tip: Sometimes it can be educational to place a breakpoint in the code and have a look to see what’s going on. As the complexity of your programs increase this will become an invaluable skill.