As you look at the building, Light is flooding in from the upper left. The picture represents this sunlight as arrows. Although the picture is 2D, try to think of the 3D scene. The direction in which the light is traveling is shown by an arrow. The length of the arrow (let us say) represents the intensity of the light. Although there are several arrows in the picture, any one arrow is enough to represent the light and its intensity.
The direction of the light and its intensity can be represented as a vector. A vector is a geometrical object that has two properties: length and direction. Light from the sun has two properties: brightness and direction (we are disregarding color information.) The length of the vector proportional to the brightness of the light.
Wind, also, can be represented as a vector. Wind has direction and speed. A particle of dust travels in the direction of the wind. The length of the vector is proportional to the speed of the dust particle.
A vector does not have a position. The picture shows several arrows that represent the flood of light coming in from the sun. But the arrows could have been drawn anywhere as long as their direction and length was the same.
Do you suspect that a computer graphics program needs to represent light intensity and direction?