Angles between flats refer to the slope of two surfaces that meet each other like the corners of a box. Imagine if you had a piece of paper and folded it so that two sides meet at a point, that point where the folds meet is an angle between the two flats. Flats are simply the surfaces of an object that are flat and not curved.
So, let's say you have a box in front of you. Each side of the box is a flat surface. Now, take your finger and run it along the edge where two sides of the box meet. You'll notice that the edge is not perfectly straight, but rather it forms an angle. That angle is the angle between flats.
The size of the angle depends on how the two sides of the box are positioned relative to each other. If the sides are at a right angle to each other (like the corners of a square), then the angle between the flats is 90 degrees. If they are at a more acute angle (less than 90 degrees) or an obtuse angle (more than 90 degrees), then the angle between flats will be a different measurement.
In summary, angles between flats refer to the slope of two flat surfaces where they meet each other. It's like the corner of a box where two sides meet and form an angle. The size of that angle depends on how the two surfaces are positioned relative to each other.