Can someone tell me what 'this' means? and why is an empty array created in this object?

Question

This is an object John my prof. created, however I didn't clearly understand what 'this' means, and why an empty array was created before the for loop.

    var john = {
    fullName: 'John Smith',
    bills: [124, 48, 180, 42],
    calcTips: function (){
        this.tips = [];
        this.finalValues = [];

        for(var i=0;i<this.bills.length;i++){
            var percentage;
            var bills = this.bills[i];

            if(bills<50){
                percentage = 0.2;
            }
            else if(bills<200&&bills>50){
                percentage = 0.15;
            }
            else{
                percentage = 0.1;
            }

                // Adds results to array
                this.tips[i] = bills * percentage;
                this.finalValues[i] = bills + (bills*percentage);
        }
    }
}