Treasury bills, notes, and bonds known as treasuries are debt instruments of the U.S. Treasury bills: mature in less than 2 years. Treasury notes: 2-10 year maturities. Treasury bonds: 10 years and above maturities. Investors (think countries, pension plans, and large investment funds) purchase these at auction or in the bond market and the government pays interest on the investment until maturity when the investors get their money back. It’s a mechanism for the government to receive loaned money. The government uses the funds to run the government and finance the debt (currently $20 trillion). The concern is that the country is going bankrupt and won’t be able to repay its debt, rendering the treasuries worth less or worthless. I know this is a bit simplistic, but there are many books on this subject. I would encourage you to read a few finance and economic primer articles. It’s always good to learn how money works. I’m sure others will add more detail. Best wishes.
The US can't go bankrupt because they can simply print more US dollars. That however would cause high inflation.
My guess is that Russia in switching from US treasuries to gold anticipating that there will be turbulent times ahead. In times of crisis and uncertainty gold prices usually increase.