Most lenders will offer a discount from their variable rate for a period of time, typically 1, 2, 3 or 5years.

Advantages:

• Payments will be lower for a period of time.
• If the variable rate goes down the payments will also reduce proportionately. 

Disadvantages:

• Payments could go up proportionately if the variable rate increases.
• There is often an Early Repayment Charge for paying more than the planned amount during the discounted term, (although some  lenders do allow limited extra payments).
• When the discount period ends, the payment is guaranteed to increase. After paying a lower rate for a few years, it can be easy to forget that you had a discount in the first place and not be prepared for the inevitable increase!
• If you want to move home during the discounted period, you may not be able to do so without being penalised (unless the loan is classed as being portable).