A border is a political line to establish the boundaries from one country to another. While seen as a colorful line on a map, borders are usually physically represented by a large wall/fence that marks the edge of the boundary. Depending on the country, borders are either highly secured or aren't enforced at all.
Not all borders are built alike or serve the same purposes. The main misconception about borders is that they're meant to keep people out. While few do represent this belief, the majority of these boundaries are fairly regulated yet welcoming depending on the home country. You could go to the Korean border for example and find an extremely contested and dangerous place. On the other hand if you go to the American-Canadian border, the potential violence wouldn't anything near what it is like in Korea.
You could compare a border to someone's property. The person who owns the property can decide who they want to let in and out at their own will. This is similar to the leader of a country deciding how much their border should be enforced. However just or unjust their decision may seem to people, the bottom line is that whoever is in charge has the power to make the decisions. My view of borders has been heavily shaped based on where I've lived my whole life. I've grown up near the very controversial Mexican-American border. Politicians have long debated whether to lift or decrease regulations on this border. This whole issue is especially important since many Mexicans poor in due to the poor economic status of Mexico in comparison to the United States. This long boundary has caused much suffering, but it has also provided protection to the country from many people who only seek to come over and do harm. This represents my personal view of the only border that I've experienced in my life.