Ok kiddo, so a long time ago, before you were even born, there was something called World War II. During this war, Spain was a neutral country which means they didn't join the bad guys or the good guys.
But even though they were neutral, lots of people still wanted Spain to help them out. One of these groups was called the Soviet Union, and they were fighting against the bad guys. The Soviet Union really needed things like guns and bullets to help them win the war, but they didn't have enough money to buy them.
So, the Soviet Union had an idea! They would give Spain some of their gold, and then Spain would give them the guns and bullets they needed. This gold that the Soviet Union gave to Spain is called "Moscow Gold" because it came from Moscow, which is a city in Russia.
The gold that the Soviet Union gave to Spain was really valuable, and it helped Spain out a lot. But, some people didn't like that Spain was exchanging this gold with the Soviet Union, because they thought it might mean that Spain was getting too involved in the war.
So, even though the gold helped both Spain and the Soviet Union, some people were not happy about it. But now that the war is over and we are all living in a peaceful world, it's important to remember how countries helped each other out during times of crisis.