A Variable Rate mortgage is one that changes when the lender announces interest rate changes. Unlike a fixed rate, if the mortgage rate goes up then you will be paying more each month. Equally if it goes down then you pay less.
Advantages:
Your monthly repayments will fall with reductions in interest rates
Gives you flexibility
Disadvantages
Your repayments will rise with interest rates
Does not give you the ability to budget for repayments as easily