When you remortgage, you are switching your mortgage to another deal, and frequently, another lender.
Remortgages can be used for various reasons. However, most people simply switch mortgages because it will work out cheaper for them. For example, the introductory interest rate may have finished with your current lender; therefore you could potentially get a new lower rate with another lender.
It is worth noting that a remortgage is not the best option in all cases. Even if the lender you are considering switching to is offering a lower rate, you must take into consideration the facts that:
The new lender may charge you for valuation and solicitors fees, even if you have already paid these for your mortgage with your current lender.
They may also charge you an arrangement fee simply for having a new deal.
Also you may be able to switch your mortgage deal with your current lender, avoiding any unnecessary costs. Many lenders will allow you to switch your mortgage deal reasonably frequently.