A track key is a programmed key that disables things like stabilitrac, traction control, changes ECM parameters. Basically it's like having a tune for your car, but only enabled with the "track key". Who's going to enable these features with the key? I'm not sure, maybe Ford. Don't know exact year it was introduced, but it was available on the 2013-14 Boss 302 and I'm sure the GT500's.
It was introduced with the Boss in 2012. It is also available for the 2013/14 GT along with the 2013 Boss. There was no 2014 Boss. It is a tune by Ford Racing.