Visa fraud is when someone lies or tricks the government into giving them permission to come into a new country. It's like if someone wants to come to your house to play but instead of asking your parents nicely, they sneak in when nobody is looking. That's not okay and it's kind of like what visa fraud is.
When grown-ups travel to new countries, they need special permission to go there called a visa. A visa is like a permission slip, just like how you need permission from your parents to go on a field trip. So if someone wants to come into a new country, they have to apply for a visa by filling out lots of paperwork and answering questions about themselves.
However, some people will try to cheat that system by telling lies or making up stories. They might say that they're a tourist when they actually want to work, or they might say that they're visiting friends when they really just want to stay in the country for a long time. When someone is caught lying or tricking the government, it's called visa fraud.
Visa fraud can be really bad because it can hurt the government and the people who live in the country. It's kind of like cheating in a game - it's not fair to the other players and can hurt how the game is played. When someone commits visa fraud, they can be punished, like if you got in trouble for cheating in a test.