What do the yellow lashes/wings on the B737 PFD indicate?
They display the maximum pitch that can be exercised with the current configuration of the aircraft. 

Using data which is measured in modern airliners, a maximum pitch angle can be calculated and displayed to the pilot on the primary display. It is described fully on this page (h/t @mins)


  With airplane aerodynamic specific data and the angle of attack measurement, we can have an effective pitch limit indicator PLI. A pitch limit indicator informs the user about the current difference between $max(\alpha)$ and the current $\alpha$. In many commercial airplane the pitch limit indicator is integrated within the glass cockpit, as shown in Figure 3; with this setup the pilot has a real-time indication of how much can pull on the column.