A standard variable rate (SVR), is the ‘normal’ rate of your lender, or their standard rate. It varies across lenders, and they can change it whenever they like.
It’s common in mortgages and normally linked to the Bank of England base rate. If the base rate goes up, lenders standard variable rate normally goes up too, and if the base rate goes down, lenders standard variable rates normally go down too. But it is up to the lender to choose to do it.
If you have a fixed rate mortgage, the standard variable rate is typically what your mortgage would move to automatically after the fixed rate period ends. And is normally a higher rate.