LDI is an acronym of Liability Driven Investing. The idea is that pension schemes use LDI to match their investments with their long-term pension liabilities. Many went about that by buying leveraged portfolios of government bonds. Famously, in September 2023, the Liz Truss/Kwasi Kwarteng budget crashed the UK government bond market and LDI magnified the problem as panicked pension funds sold their bonds. In the end the Bank of England had to step in to provide liquidity and calm markets.