Most of the tool have a description on the right side of the window (if not visible click on the arrow in the upper right corner)
In your case the description read :
This algorithm takes a line or polygon layer and generates a point layer with points representing the vertices in the input lines or polygons. The attributes associated to each point are the same ones associated to the line or polygon that the point belongs to.
Additional fields are added to the point indicating the vertex index (beginning at 0), the vertex’s part and its index within the part (as well as its ring for polygons), distance along original geometry and bisector angle of vertex for original geometry.
So it seem that the angle is the
bisector angle of vertex for original geometry
I did compare the result with the "angle_at-vertex" expression and I get the same angle for each vertex than those from the Extract vertices tool.
The ""angle_at-vertex" description read :
Returns the bisector angle (average angle) to the geometry for a specified vertex on a linestring geometry. Angles are in degrees clockwise from north.
My understanding is that the angle you get at vertex n is the angle between North and the line geometry between vertex n and n+1 in degrees clockwise from north.
Your data doesn't seem to really follow that, could it be related to projection ?
Angle value on point is average angle of lines before and after vertex.
First point has no "before line" so its actual angle of line after vertex. Last point has no "after line" so its actual angle of line before vertex.
Angle is in degrees clockwise from north to line geometry.