Imagine you are outside in the field, where you can see the horizon. You could make a map of the complete horizon around you. This map can be divided in 360 degrees, because the horizon can be seen as a circle of 360 degrees around you.
The next picture is made with a normal camera. It shows a part of the horizon with the Moon. The width of the picture is ± 40 degrees.
If we zoom in on the Moon with a telelens mounted on the camera, the picture we will get has a smaller field of view (angle of the image width). In this picture you can see that the angle of the Moon is ± 0.5 degrees. This is called the angular diameter of the Moon.
One degree can be divided in 60’ (arcminutes), and one arcminute can be divided in 60" (arcseconds).