There are 360 degrees in a circle. A "degree" here is not a size of something, it is a mark of progress in travelling around the edge of the circle. If you stand inside a circle facing one way, then turn to face a different way, there will be a certain number of degrees on the circle's edge between where you were facing and where you are facing now. This number of degrees is the ANGLE between the two directions.
The angles in every triangle add up to 180 degrees, while the angles in every rectangle or square total 360 degrees.