The angle the two arrows open up from a shared start is the angle between them, and the shadow one casts onto the other is its projection.
Module goal. Introduce the operation that multiplies two vectors into a number, capturing the angle between them, projection, and when they're perpendicular.
eastmath.com · 27.4 The Dot Product: Angles and Perpendicularity · 27.4.1 The Angle Between Vectors and Projection