Radio jamming is when someone purposely creates interference on the radio waves so that people can't hear what's being said on the radio. It's like when you try to talk to your friend on a walkie-talkie, but someone else keeps making loud noises so you can't hear them.
In Korea, some people are jamming the radio signals, so North Koreans can't listen to the outside world. North Koreans are not allowed to listen to anything that goes against their government. But sometimes people in North Korea want to know what's going on outside of their country. So they try to listen to radio broadcasts from other countries, like South Korea or the United States.
But the people who jam the radio signals don't want North Koreans to be able to listen to these broadcasts. So they make it so that the radio signals are all jumbled up and hard to hear. This way, North Koreans don't know what's happening outside of their country, and they can only hear what their government allows them to listen to.