In the picture, we see 8 people waiting in line to buy movie tickets. If more people come to the movies, the line will be longer. If there were few people, the line would be smaller.
The line length and the number of people are dependent on each other, that is, they are related.
When a thing depends on another, we say that they are related.
But the number of people in the line and the number of cars on the street are not related. In this case, the quantities are independent.