UK finance jargon - in plain English
The percentage charged for borrowing, or earned on savings.
An interest rate is a percentage applied to a sum of money. It's either what you pay to borrow (on a loan, credit card, or mortgage) or what you earn by saving (in a bank account). In the UK, the Bank of England sets a base rate that influences all other rates in the economy. When the base rate rises, mortgage rates and savings rates tend to follow, though not always at the same speed or by the same amount.
The Bank of England base rate influences what banks charge each other to borrow money. Your savings account will typically offer a rate somewhere near the base rate, while your mortgage rate will sit above it. The lender adds a margin on top. Banks earn part of their income from the gap between what they charge borrowers and what they pay savers.
Numbers simplified for illustration. Actual rates vary by product, lender, and the Bank of England's current base rate.
People assume savings rates and mortgage rates move together in lockstep when the base rate changes. In practice, banks tend to pass rate rises on to mortgages faster than they do to savings accounts. When rates fall, the reverse often applies. It's worth actively shopping around rather than assuming your current account or mortgage is still competitive.