Export restriction is a rule that says you cannot send certain things to certain places. For example, imagine you have a toy car that you really like. One day, your friend in another country really wants that toy car, but your parent tells you that you cannot give it to them. This is because your parent wants to keep the toy car in your house, and not send it away to your friend.
Similarly, countries have things that they make or grow that they want to keep for themselves. If they think that sending these things somewhere else will not be good for their own country, they might make a rule that says they cannot export those things. The things they might restrict could be materials like oil, technology or weapons.
Export restrictions can also be put in place for political reasons. If two countries are having a conflict or argument, one country might not want to send things to the other country. This is like when you and your friend get into an argument and you do not want to play with them for a while.
Overall, export restriction is like a rule that makes sure things do not leave a place if it would not be good for that place or its people.