Answer:
When we talk about an extended source, we mean a source that is not a point source but has some finite size. Examples of extended sources include the Sun, light bulbs, and other sources that are not point-like. When we observe an extended source from a large distance, it appears to behave as a point source.
The reason for this has to do with the fact that at large distances, the size of the extended source becomes very small in comparison to the distance between the observer and the source. This means that the light rays that reach the observer from different parts of the extended source are almost parallel to each other. As a result, the light from the different parts of the source overlaps and interferes with each other in such a way that the extended source appears to be a single point. This effect is called diffraction, and it is the same effect that causes waves to bend around obstacles and spread out after passing through a narrow opening.
So, in summary, an extended source seems to behave as a point source at large distances because of diffraction, which causes the light from the different parts of the source to overlap and interfere with each other in a way that makes the source appear to be a single point.