Discount
Your monthly payments can go up or down, but you get a discount on the lender's standard variable rate for a set period of time. At the end of the deal, you usually change over to the standard variable rate.
There are almost always penalties during the special deal period. They can apply even after the end of the special deal period as well.
It gives you a gentler start to your mortgage, at a time when money may well be tight. But you must be confident you can afford the payments when the discount ends.
The discount period is limited, so don't get used to those early low repayments.
You may not be able to make overpayments and pay off the loan early without penalties
The lender may not reduce, or may delay reducing their variable rate even if the Bank of England rate goes down