A product transfer is when you move to a new mortgage deal with your existing lender at the end of your current deal, rather than remortgaging to a different lender. The loan stays with the same lender, the amount borrowed and the term normally stay the same, and the property does not change hands, so it is a switch of product rather than a new mortgage.
Because there is no change of lender, a product transfer usually avoids a new valuation and legal work, and lenders generally do not require full affordability reassessment where the borrowing is not increasing. That can make it a quicker route than a full remortgage, particularly if your circumstances have changed since you first applied. The trade-off is that you only see one lender’s range, so it is worth comparing the transfer offer against the wider market before accepting it.