Unfamiliar syntax for JavaScript object key

Question

I've come across an object written like this:

 var box = {
        locked: true,
        unlock: function () { this.locked = false; },
        lock: function () { this.locked = true; },
        _content: [],
        get content() {
            if (this.locked) throw new Error("Locked!");
            return this._content;
        }
    };

I've never seen 'get content()' before. What does this represent? Is there a key in there somewhere? Or is 'get' a keyword that performs some action on the proceeding block of code? Thanks!