A Vertex is just a corner of a Polygon (Triangle, Rectangle, Trapezoid, etc.) or the end of a Line Segment or Arrow. The Octagon below has 8 vertices.
Conventionally, in mathematics, the ends of a Line Segment wouldn’t be considered vertices, but in Doodleback they are. It’s important to know what’s considered a Vertex and what isn’t for purposes of applying Vertex Tools to them. The Vertex Tools allow you to move vertices. In the image below we’re using the Move Vertex (Warp) tool to move the vertex of the Octagon above.
We could also have moved the Vertex without warping the sides, or we could have moved a collection of vertices simultaneously too–with or without warping. See the Vertex Tools for more info.