What it means: The IMF (aka the International Monetary Fund, which sort of functions like the world’s piggy bank) has put together a league table of how strong government finances are around the world, and, well, it turns out our government suck at it. (They did beat out Portugal though. Go figure).
The problem is that the UK government doesn’t have many assets compared to its liabilities. Assets are anything of value: oil reserves, railway companies, piles of gold doubloons, etc. Liabilities are anything they need to pay for: debts, pensions, orders for American F-35 fighter jets, and so on.
When we say the UK has “not many” assets, we mean in rich-country government terms - it still has £3 trillion worth, which is hardly petty cash. The government’s liabilities, however, are a cool £5 trillion, and it’s this £2 trillion shortfall that makes the IMF regard the UK government as less wealthy than most.
The UK has fewer assets than most countries because we’ve privatised (sold off) a lot of them, including our energy and water companies, railways, airports and postal service. Our liabilities come mostly from having lots of government debt (we were the joint 19th most indebted country in the world in 2017, an honour we shared with war-torn Yemen) and lots of state pensions to pay out (basically, we’ve got more and more old people and they’re living longer).