What does this symbol mean in PCB Layout?
It's a drawing artifact with no electrical meaning.  

If you look at the Gerber file you send to the PCB manufacturer, you will find those line segments are made by moving a circular aperture along a set of coordinates. 

And in the old days of "photoplotting" that is exactly how it worked. A bright light shone through the aperture onto photographic film. If the aperture stayed in one place, you got a circle. (This is useful for pads, vias, drill holes etc of that diameter). 

If it moved in a straight line you got a rectangle with rounded ends. As Uwe says, a round end allows direction changes at any angle to look good and preserve the trace width.

What you see on screen is your graphics card getting very slightly confused about how to draw two circles (representing the end of one line segment, and the start of another) on top of each other.