A border is where something ends and something new begins. Usually, when you think of a border you think of a border between two different regions. That can be a border between two countries, for example, Germany and Austria.
If you travel from Germany to Austria, you cross an international border. You are not in the German territory anymore, where the German laws apply, but in the Austrian territory, where you need to follow Austrian rules. Germany and Austria are both part of the European Union. A lot of laws apply to both countries. People can easily travel between both countries.
On other borders, the state watches, who or what crosses the border. Maybe criminals are trying to escape from the police. In foreign countries, the police can't chase them anymore, unless they ask the police in the other country for help. Many states don't want goods to get in without control. Because in some states, goods are cheaper than in others. Meat is cheaper in Germany than in Switzerland. If you bring a big amount of meat from Germany to Switzerland, you need to pay for customs.
How can you recognize a border?
There are borders that are so well protected that you cannot miss them. For example, the US-Mexico border. This border has big fences and at some points, it is fortified with walls. This is because massive amounts of refugees are trying to get into the US. The Soviet Union was also well protected but in a different way. Their borders were there to keep people inside the country.
It is common that there is a street that crosses the border, but these streets have barriers and you have to pass through customs to enter the other country. Oftentimes, you need to show your passport.
An officer might ask you if you have anything in your belongings worth mentioning if so, you might have to pay a bill for these objects. It is also possible that you need a visa to cross a border. A visa is a paper that allows a person to travel to another country, for example, three months, then you have to leave again.
When you talk about a Green Border, you are talking about a border that is in nature. If you look on a map, you can see the border, in real life, you can’t see the border if you walk through the woods.
Often there is a landscape that makes borders more visible like rivers and mountains. For example, the Rhine in Germany is a massive river that separates Germany from France. There are also borders that are on the sea. These borders belong to nearby states. But lots of states argue about these borders because they are unsure about where the borders lie.