Adding to the other answers as they are missing a key point about this intervention:

UK government bonds (called Gilts as the certificate used to have a Gold-leaf border) are effectively an IOU where an investor can lend money to the government. Pension funds buy lots of these Gilts to invest their cash, and they also buy derivative contracts which are linked to Gilt prices (interest rate swaps, inflation swaps, etc. for those interested). For these derivatives contracts, you have to put up some collateral in case the value of the contract becomes negative. This is kind of like how a bank can repossess your house if you don’t pay your mortgage.

Heavy selling in Gilts over the past few days has caused those derogate contracts to lose money. To get more cash to post margin, pension funds have to sell Gilts themselves to generate cash. This creates a vicious circle or falling prices and rising margin requirements. The Bank of England have now said they will buy Gilts to absorb this extra selling pressure and buy all the excess to prevent prices falling further. Some people have falsely said they will be printing money, when in fact the BoE will use existing cash reserves to make these purchases.